
For a spatial twisted central configuration of the Newtonian (2N+1)-body problem where 2N masses are at the vertices of two paralleled regular N-polygons with distance h>0, and the twist angle between the two regular N-polygons is 0≤θ<2π, we study the sufficient and necessary conditions for the existence of the spatial twisted central configuration. Additionally, we obtain the uniqueness of the spatial twisted central configuration.
Citation: Liang Ding, Jinrong Wang, Jinlong Wei. Spatial twisted central configuration for Newtonian (2N+1)-body problem[J]. Communications in Analysis and Mechanics, 2024, 16(2): 388-415. doi: 10.3934/cam.2024018
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For a spatial twisted central configuration of the Newtonian (2N+1)-body problem where 2N masses are at the vertices of two paralleled regular N-polygons with distance h>0, and the twist angle between the two regular N-polygons is 0≤θ<2π, we study the sufficient and necessary conditions for the existence of the spatial twisted central configuration. Additionally, we obtain the uniqueness of the spatial twisted central configuration.
For the spatial Newtonian n-body problem, the equations of motion for the n masses mk>0 and positions xk∈R3 with k∈{1,…,n} can be described by Newton's second law and Newton's universal gravitation law:
mk¨xk=∂(∑1≤s<j≤nmjms|xj−xs|)∂xk. |
Define
Ω={x: x=(x1,x2,…,xn)∈(R3)n}, |
and let
△=⋃1≤j≠s≤n{x=(x1,x2,…,xn)|xj=xs,1≤j≠s≤n} |
be the collision set. As usual, the set Ω∖△ is called the configuration space. First, we introduce the definition of central configuration for the Newtonian n-body problem (see [1]).
Definition 1.1. Given n masses mk>0 with positions qk∈R3, k=1,…,n, we say a configuration q∈Ω∖△ is a central configuration at some moment if there exists a constant λ∈R such that
{∑j≠k1≤j≤nmjmk(qj−qk)|qj−qk|3=−λmk(qk−x0), k=1,2,…,n,x0=∑1≤k≤nmkqk∑1≤k≤nmk. | (1.1) |
Central configurations play a very important role in the study of the Newtonian n-body problem, and especially, central configurations can lead to rigid-motion solutions and homothetically collapsing solutions [1]. Central configurations of the Newtonian three-body (n=3) problem with any given three masses have long been known, and there are always exactly two kinds of central configurations: Euler collinear central configuration and Lagrange equilateral-triangle central configuration [2,3]. For a planar Newtonian N-body problem with n=N≥4, Perko and Walter [4] proved that if N masses are located at the vertices of a regular N-polygon (see Figure 1), then they can form a regular polygonal central configuration if and only if all the values of N masses are equal to each other. For more results of planar central configuration with one regular N-polygon, one can refer to [5,6,7,8].
For a planar central configuration with n=2N and N≥2 such that two regular N-polygons are concentric and that 2N equal masses are placed at the vertices of the two regular N-polygons (see Figure 2), Zhang and Zhou [9] proved that the values of masses in each separate regular N-polygon were equal. We say that p regular N-polygons with p≥2 are nested if they are coplanar and have the same number of vertices N and the same center, and the positions of the vertices of the innermost regular N-polygon R(1)j and those of the remaining p-1 regular N-polygons R(k)j with any k∈{2,…,p} satisfy the relation that R(p)j=s1R(p−1)j=s2R(p−2)j=…=sp−1R(1)j for some scale factors sp−1>sp−2>…>s1>1 and for all j=1,2,…,N. For the central configuration such that two regular N-polygons are nested, masses on different regular N-polygons may be different, and Moeckel and Simó [10] proved that for every mass ratio b between the two masses, there were exactly two planar central configurations. Also, for the case of n=2N such that N equal masses are placed at the vertices of one regular N-polygon and the remaining N equal masses are placed at the vertices of the other regular N-polygon, which is rotated exactly at an angle θ=π/N with respect to the former regular N-polygon, Barrabés and Cors [11] proved the existence of the planar central configuration with any value of the mass ratio. For the case of n=pN and p≥2, Corbera, Delgado, and Llibre [12] proved the existence of the nested central configuration such that pN masses were at the vertices of the p nested regular N-polygon with a common center. Moreover, all the masses on the same regular N-polygon were equal, but masses on a different regular N-polygon could be different. For the case of n=pN+gN with p≥1 and g≥1, Zhao and Chen [13] proved the existence of planar central configurations such that p regular N-polygons were nested, and g regular N-polygons were rotated exactly at an angle π/N with respect to the other ones. For more details in this direction, we refer to [14,15,16,17,18,19,20,21] and the references therein.
Note that for a planar central configuration with n=N+1, Chen and Luo [22] proved that if N masses are located at the vertices of one regular N-polygon and the position of the (N+1)-th mass is on the plane containing the regular N-polygon (see Figure 3), then all the values of N masses located at the vertices of the regular N-polygon are equal to each other. For a spatial central configuration with n=N+1 and the (N+1)-th mass off the plane containing the regular N-polygon (see Figure 4), Ouyang, Xie, and Zhang [23] showed that the distance between the (N+1)-th mass and the regular N-polygon was unique.
In this paper, we consider the spatial central configuration of a Newtonian (2N+1)-body problem in R3 formed by 2N masses placed at the vertices of two paralleled regular N-polygons with distance h>0. It is assumed that the lower layer regular N-polygon lies in a horizontal plane, and the upper regular N-polygon parallels the lower one in R3 with distance h, and the z-axis passes through both centers of the two regular N-polygons (see Figure 5). For convenience, when choosing the coordinates, we treat R3 as the direct product of the complex plane and real axis. For the positions of the 2N+1 masses q=(q1,q2,…,q2N,q2N+1)∈Ω∖△, we have
{qk=(ρk, 0), k=1,…,N,ql=(aρleiθ, h), 0≤θ<2π, l=N+1,…,2N, a>0, h>0, | (1.2) |
where a is the ratio of the sizes of the two regular N-polygons, ρd is the d(modN)-th complex root of unity, i.e., ρk=eiθk with k=1,2,…,N, and ρl=eiθl with l=N+1,N+2,…,2N and θd=2dπ/N with d∈Z. Here, we define θ as the twist angle between the two paralleled regular N-polygons with distance h>0. Moreover, for the position of the (2N+1)-th mass and the barycenter x0=(c0,h0), we define
{q2N+1=(a1eiα, h2N+1), a1≥0, 0≤α<2π, −∞<h2N+1<+∞,c0=∑1≤k≤Nmkρk+∑N+1≤l≤2Namlρleiθ+a1m2N+1eiαm2N+1+∑1≤k≤Nmk+∑N+1≤l≤2Nml,h0=∑N+1≤l≤2Nmlh+m2N+1h2N+1m2N+1+∑1≤k≤Nmk+∑N+1≤l≤2Nml. | (1.3) |
Then, for the spatial twisted configuration with n=2N+1 and the notations (1.2)–(1.3), we have the following results.
For the existence, we have the following:
Theorem 1.1. Suppose the values of N masses with N≥2 located at the vertices of one regular N-polygon are equal to each other, and all of the sides in the two regular N-polygons have the same size. Define the position of q2N+1 as (1.3). Then, the 2N+1 masses form a central configuration if and only if all the values of the 2N masses located at the vertices of the two regular N-polygons are equal to each other, a1=0 and h2N+1=h/2, and the twist angle is θ=sπ/N with s∈{0,1,…,2N−1}.
Remark 1.1. For the spatial twisted central configuration of the Newtonian 2N-body problem, under the assumption that the values of masses in each separate regular N-polygon were equal, Yu and Zhang [24] proved that the twist angle must be θ=0 or θ=π/N (see Figures 6 and 7). Meanwhile, in Theorem 1.1, we consider the spatial twisted central configuration of the Newtonian (2N+1)-body problem. Under the assumptions that the values of the N masses located at the vertices of one regular N-polygon are equal and that all of the sides in the two regular N-polygons have the same size, we not only obtain the values of the twist angle; but also prove that all 2N masses must be equal. Moreover, we know the position of the (2N+1)-th mass is (0,0,h/2).
For the uniqueness, we have the following:
Theorem 1.2. For the spatial twisted central configuration, if the values of the N masses located at the vertices of one regular N-polygon are equal to each other and all of the sides in the two regular N-polygons have the same size, then for any N≥2, both the distance between the two regular N-polygons and the position of the (2N+1)-th mass are unique.
Lemma 2.1. [24, Lemma 2.9] For any a>0, any γ∈(−∞,+∞), any h>0, and any N≥2, let
f(γ)=∑1≤j≤Nasin(θj+γ)[1+a2−2acos(θj+γ)+h2]32. | (2.1) |
Then,
f(πN)=0, f(−γ)=−f(γ) and f(γ+2πN)=f(γ). |
Remark 2.1. In Lemma 2.1, if we choose a=1 and γ=π/N where N≥2, then
∑1≤j≤Nsin(θj−πN)[2−2cos(θj−πN)+h2]32=∑1≤j≤Nsin(θj+πN)[2−2cos(θj+πN)+h2]32=0, where h>0. |
Lemma 2.2. [24, Lemma 2.10] If γ∈(0,π/N) with N≥2, then for any a>0 and any h>0, we have f(γ)>0.
Lemma 2.3. If θ=sπ/N with s∈{0,1,…,2N−1} and N≥2, then for any k′∈{1,2,…,N}, any l′∈{N+1,N+2,…,2N}, and any h>0, we have
{∑1≤k≤Nei(θk−l′−θ)−1[|ei(θk−l′−θ)−1|2+h2]32=∑N+1≤l≤2Nei(θl−k′+θ)−1[|ei(θl−k′+θ)−1|2+h2]32=∑1≤j≤Nei(θj+θ)−1[|ei(θj+θ)−1|2+h2]32∈R,∑1≤k≤Nh[|ei(θk−l′−θ)−1|2+h2]32=∑N+1≤l≤2Nh[|ei(θl−k′+θ)−1|2+h2]32=∑1≤j≤Nh[|ei(θj+θ)−1|2+h2]32. | (2.2) |
Proof. Let ˆμ∈{1,2,…,N}, ˜μ∈{0,1,…,N−1}, ˜α∈(−2π,2π), and κ∈{0,1}. We define a mapping σ by
{{κN+1,κN+2,…,κN+N}σ⟶{˜α,2πN+˜α,...,2(N−1)πN+˜α},σ(μ)=[(μ−ˆμ+˜μ)(modN)]2πN+˜α,∀μ∈{κN+1,κN+2,…,κN+N}. | (2.3) |
Notice that both {κN+1,κN+2,…,κN+N} and {˜α,(2π)/N+˜α,…,2(N−1)π/N+˜α} are finite sets; the mapping σ is a surjection. Let us show σ is an injective mapping. The proof for κ=0 is similar to κ=1; we only check for κ=1. Let μ1≠μ2 and μ1,μ2∈{N+1,N+2,…,2N}. If σ(μ1)=σ(μ2), then there exist s1,s2∈Z such that
(μ1−ˆμ+˜μ)+s1N=(μ2−ˆμ+˜μ)+s2N. | (2.4) |
Hence, μ1−μ2=(s2−s1)N. By the facts that −N<μ1−μ2<N and s2−s1∈Z, s2=s1, and thus μ1=μ2, which is a contradiction. Therefore, σ is injective, which implies that σ is a bijection.
Similarly, for l∈{N+1,N+2,…,2N}, ˜s∈{0,1,…,N−1}, and θ∈[0,2π), if we define another mapping σ1 by
{{1,2,…,N}σ1⟶{θ,2πN+θ,…,2(N−1)πN+θ},σ1(k′)=[(l−k′+˜s)(modN)]2πN+θ,∀k′∈{1,2,…,N}, | (2.5) |
then σ1 is a bijection as well. Together with the fact that θ=sπ/N with s∈{0,1,…,2N−1} is equivalent to θ=2˜sπ/N or θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}, let us show (2.2) holds by considering the following two cases.
Case 1. θ=2˜sπ/N with ˜s∈{0,1,…,N−1}.
Observing that θj=2jπ/N with j∈{1,2,…,N},
∑1≤j≤Nsinθj[2−2cosθj+h2]32=∑1≤j≤Nsin2πN−jN[2−2cos2πN−jN+h2]32=−∑1≤j≤Nsinθj[2−2cosθj+h2]32, |
which implies that
∑1≤j≤Nsinθj[2−2cosθj+h2]32=0. | (2.6) |
Let k′∈{1,2,…,N} and ˜s∈{0,1,…,N−1}. In (2.3), if we choose μ=l, ˆμ=k′, ˜μ=˜s, κ=1, and ˜α=0, then the mapping
{{N+1,N+2,…,2N}σ⟶{0,2πN,…,2(N−1)πN},σ(l)=[(l−k′+˜s)(modN)]2πN, ∀l∈{N+1,N+2,…,2N} |
is a bijection. Combining (2.6), and θd=2πd/N with d∈Z and
{cos(θl−k′+2πN˜s)=cos(2πN[(l−k′+˜s)(modN)]),sin(θl−k′+2πN˜s)=sin(2πN[(l−k′+˜s)(modN)]) |
for any k′∈{1,2,…,N} and any ˜s∈{0,1,…,N−1}, we have
{∑N+1≤l≤2Nsin(θl−k′+2πN˜s)[2−2cos(θl−k′+2πN˜s)+h2]32=∑N+1≤l≤2Nsinθl−k′+˜s[2−2cosθl−k′+˜s+h2]32=∑1≤j≤Nsinθj[2−2cosθj+h2]32=0,∑N+1≤l≤2Ncos(θl−k′+2πN˜s)−1[2−2cos(θl−k′+2πN˜s)+h2]32=∑N+1≤l≤2Ncos(θl−k′+˜s)−1[2−2cos(θl−k′+˜s)+h2]32=∑1≤j≤Ncosθj−1[2−2cosθj+h2]32,∑N+1≤l≤2Nh[2−2cos(θl−k′+2πN˜s)+h2]32=∑N+1≤l≤2Nh[2−2cos(θl−k′+˜s)+h2]32=∑1≤j≤Nh[2−2cosθj+h2]32. | (2.7) |
On the other hand, in (2.3), for any l′∈{N+1,N+2,…,2N} and any ˜s∈{0,1,…,N−1}, if we let μ=k, ˆμ=l′, ˜μ=N−˜s, κ=0, and ˜α=0, then we know that the mapping
{{1,2,…,N}σ⟶{0,2πN,…,2(N−1)πN},σ(k)=[(k−l′+(N−˜s))(modN)]2πN=[(k−l′−˜s)(modN)]2πN, ∀k∈{1,2,…,N} |
is a bijection as well. Thus, similar to the procedure of obtaining (2.7), for any l′∈{N+1,N+2,…,2N}, any ˜s∈{0,1,…,N−1}, and any h>0, we have
{∑1≤k≤Nsin(θk−l′−2πN˜s)[2−2cos(θk−l′−2πN˜s)+h2]32=∑1≤j≤Nsinθj[2−2cosθj+h2]32=0,∑1≤k≤Ncos(θk−l′−2πN˜s)−1[2−2cos(θk−l′−2πN˜s)+h2]32=∑1≤j≤Ncosθj−1[2−2cosθj+h2]32,∑1≤k≤Nh[2−2cos(θk−l′−2πN˜s)+h2]32=∑1≤j≤Nh[2−2cosθj+h2]32. | (2.8) |
Employing (2.7), (2.8), and θd=2πd/N with d∈Z, we have
∑N+1≤l≤2Nei(θl−k′+2πN˜s)−1[|ei(θl−k′+2πN˜s)−1|2+h2]32=∑N+1≤l≤2N[cos(θl−k′+2πN˜s)−1]+isin(θl−k′+2πN˜s)[2−2cos(θl−k′+2πN˜s)+h2]32=∑1≤j≤N(cosθj−1)+isinθj[2−2cosθj+h2]32=∑1≤j≤Ncosθj−1[2−2cosθj+h2]32=∑1≤k≤N[cos(θk−l′−2πN˜s)−1]+isin(θk−l′−2πN˜s)[2−2cos(θk−l′−2πN˜s)+h2]32=∑1≤k≤Nei(θk−l′−2πN˜s)−1[|ei(θk−l′−2πN˜s)−1|2+h2]32∈R |
and
∑N+1≤l≤2Nh[|ei(θl−k′+2πN˜s)−1|2+h2]32=∑N+1≤l≤2Nh[2−2cos(θl−k′+2πN˜s)+h2]32=∑1≤j≤Nh[2−2cosθj+h2]32=∑1≤j≤Nh[2−2cosθj+h2]32=∑1≤k≤Nh[2−2cos(θk−l′−2πN˜s)+h2]32=∑1≤k≤Nh[|ei(θk−l′−2πN˜s)−1|2+h2]32, |
where ˜s∈{0,1,…,N−1}, k′∈{1,2,…,N}, and l′∈{N+1,N+2,…,2N}. Thus, (2.2) holds for the case of θ=2˜sπ/N with ˜s∈{0,1,2,…,N−1}.
Case 2. θ=2˜sπ/N+π/N with ˜s∈{0,1,2,…,N−1}.
For any l′∈{N+1,N+2,…,2N} and any ˜s∈{0,1,…,N−1}, in (2.3), if we choose μ=k, ˆμ=l′, ˜μ=N−˜s, κ=0, and ˜α=−π/N, then the mapping
{{1,2,…,N}σ⟶{−πN,2πN−πN,…,2(N−1)πN−πN},σ(k)=[(k−l′+(N−˜s))(modN)]2πN−πN=[(k−l′−˜s)(modN)]2πN−πN, ∀k∈{1,2,…,N} |
is a bijection. Then, we have
{∑1≤k≤Nsin(θk−l′−2πN˜s−πN)[2−2cos(θk−l′−2πN˜s−πN)+h2]32=∑1≤k≤Nsin(θk−l′−˜s−πN)[2−2cos(θk−l′−˜s−πN)+h2]32=∑1≤j≤Nsin(θj−πN)[2−2cos(θj−πN)+h2]32,∑1≤k≤Ncos(θk−l′−2πN˜s−πN)−1[2−2cos(θk−l′−2πN˜s−πN)+h2]32=∑1≤k≤Ncos(θk−l′−˜s−πN)−1[2−2cos(θk−l′−˜s−πN)+h2]32=∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32, | (2.9) |
where ˜s∈{0,1,…,N−1}, l′∈{N+1,N+2,…,2N}, and h>0.
By the first equation of (2.9) and Remark 2.1, for any ˜s∈{0,1,…,N−1}, any l′∈{N+1,N+2,…,2N}, and any h>0, we see that
∑1≤k≤Nsin(θk−l′−2πN˜s−πN)[2−2cos(θk−l′+2πN˜s−πN)+h2]32=∑1≤j≤Nsin(θj−πN)[2−2cos(θj−πN)+h2]32=∑1≤j≤Nsin(θj+πN)[2−2cos(θj+πN)+h2]32=0. | (2.10) |
Moreover, in (2.5), for any l∈{N+1,N+2,…,2N} and any ˜s∈{0,1,…,N−1}, if we choose θ=π/N, then the mapping
{{1,2,…,N}σ1⟶{πN,2πN+πN,…,2(N−1)πN+πN},σ1(k′)=[(l−k′+˜s)(modN)]2πN+πN, ∀k′∈{1,2,…,N} |
is a bijection. Hence, combining (2.10), this leads to
{∑1≤k′≤Nsin(θl−k′+2πN˜s+πN)[2−2cos(θl−k′+2πN˜s+πN)+h2]32=∑1≤k′≤Nsin(θl−k′+˜s+πN)[2−2cos(θl−k′+˜s+πN)+h2]32=∑1≤j≤Nsin(θj+πN)[2−2cos(θj+πN)+h2]32=0,∑1≤k′≤Ncos(θl−k′+2πN˜s+πN)−1[2−2cos(θl−k′+2πN˜s+πN)+h2]32=∑1≤k′≤Ncos(θl−k′+˜s+πN)−1[2−2cos(θl−k′+˜s+πN)+h2]32=∑1≤j≤Ncos(θj+πN)−1[2−2cos(θj+πN)+h2]32. | (2.11) |
Furthermore, one can verify that
∑1≤j≤Ncos(θj+πN)−1[2−2cos(θj+πN)+h2]32−∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32=∑2≤j≤N+1cos(θj−πN)−1[2−2cos(θj−πN)+h2]32−∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32=∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32−∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32=0, |
and this implies that
∑1≤j≤Ncos(θj+πN)−1[2−2cos(θj+πN)+h2]32=∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32. | (2.12) |
Employing (2.9), (2.10), (2.11), and (2.12), for any l,l′∈{N+1,N+2,…,2N} and any ˜s∈{0,1,…,N−1}, we have
{∑1≤k≤Ncos(θk−l′−2πN˜s−πN)−1[2−2cosθ(k−l′−2πN˜s−πN)+h2]32=∑1≤k≤Ncos(θl−k′+2πN˜s+πN)−1[2−2cosθ(l−k′+2πN˜s+πN)+h2]32=∑1≤j≤Ncos(θj−πN)−1[2−2cos(θj−πN)+h2]32,∑1≤k≤Nsin(θk−l′−2πN˜s−πN)[2−2cos(θk−l′−2πN˜s−πN)+h2]32=∑1≤k≤Nsin(θl−k′+2πN˜s+πN)[2−2cos(θl−k′+2πN˜s+πN)+h2]32=∑1≤j≤Nsin(θj−πN)[2−2cos(θj−πN)+h2]32=0. |
Thus, (2.2) holds for the case of θ=2˜sπ/N+π/N, where ˜s∈{0,1,2,…,N−1}.
By Cases 1–2, we arrive at the conclusion that (2.2) holds for θ=sπ/N with s∈{0,1,…,2N−1}.
Remark 2.2. Similar to dealing with the mappings σ and σ1, for any k′∈{1,2,…,N} and any l′∈{N+1,N+2,…,2N} with N≥2, if one defines σ2 and σ3 by
{{1,2,…,N}∖{k′}σ2⟶{2πN,4πN,…,2(N−1)πN},σ2(k)=[(k−k′)(modN)]2πN, ∀k∈{1,2,…,N}∖{k′}, |
and
{{N+1,N+2,…,2N}∖{l′}σ3⟶{2πN,4πN,…,2(N−1)πN},σ3(l)=[(l−l′)(modN)]2πN, ∀l∈{N+1,N+2,…,2N}∖{l′}, |
then σ2 and σ3 are bijections.
Lemma 2.4. For any θ∈R, N≥2, a>0, h>0, and m>0, we have
∑1≤k≤N−hm[|eiθk−aei(θl′+θ)|2+h2]32≡constant, ∀l′∈{N+1,N+2,…,2N}. | (2.13) |
Proof. In fact, (2.13) is equivalent to
∑1≤k≤N−h[|ei(θk−l′−θ)−a|2+h2]32≡constant, ∀l′∈{N+1,N+2,…,2N}. |
It is easy to see that k−l′∈Z. Moreover, in (2.3), for any l′∈{N+1,N+2,…,2N} and any θ∈[0,2π), if we let μ=k, ˆμ=l′−N, ˜μ=0, κ=0, and α=θ, then the mapping
{{1,2,…,N}σ⟶{θ,2πN+θ,…,2(N−1)πN+θ},σ(k)=[(k−(l′−N))(modN)]2πN+θ=[(k−l′)(modN)]2πN+θ, ∀k∈{1,2,…,N} |
is a bijection, which implies that
∑1≤k≤N−h[|ei(θk−l′−θ)−a|2+h2]32=∑1≤j≤N−h[|ei(θj−θ)−a|2+h2]32, ∀l′∈{N+1,N+2,…,2N}. |
Thus, Lemma 2.4 is true.
Lemma 2.5. [4, Page 304] For any N≥2, ∑1≤j≤N−1(1−eiθj)/|1−eiθj|3=[∑1≤j≤N−1csc(jπ/N)]/4.
Lemma 2.6. [20, Pages 1431 and 1437] For any N≥2, the inequality
∑1≤j≤N1+cos(θj+πN)[2−2cos(θj+πN)]32−∑1≤j≤N−11−eiθj|1−eiθj|32>0 |
holds.
Now, we introduce the definition of circulant matrix and state its properties.
Definition 2.1. [25, Pages 65–66] A matrix ˜C=(˜ckj)N×N is circulant if ˜cˆk,ˆj=˜cˆk−1,ˆj−1 where 1≤k,j,ˆk,ˆj≤N and N≥2.
In Definition 2.1, we take the circulant matrix ˜C as the following:
˜C=:C=(ck,j), where ck,j={1−ρk−j|1−ρk−j|3,k≠j,0,k=j. | (2.14) |
We have some properties for the special circulant matrix C.
Lemma 2.7. [4, Page 303] The circulant matrix C has the same forms of the eigenvalues λj(C) and the corresponding eigenvectors ξj; more precisely,
λj(C)=∑1≤k≤Nc1,kρk−1j−1,ξj=(ρj−1,ρ2j−1,…,ρNj−1)T,j=1,2,…,N, |
where N≥2 and ρj−1=eiθj−1=e2(j−1)πi/N.
Lemma 2.8. [4, Corollary and Lemma 12] For the eigenvalues of C with j≠N and N≥4, λj≠0 except that λ(N+1)/2=0 for odd N.
Lemma 2.9. [22, Proposition 2.2] The eigenvectors ξj (j=1,2,…,N and N≥3) of circulant matrix C form a basis of CN.
Lemma 2.10. [25, Page 65] Denote the conjugate transpose of νk by (ˉνk)T. Then,
(ˉξk)Tξj={N, k=j,0, k≠j, (ρ−1,ρ−2,…,ρ−N)(ˉξN)T=N. |
Let k′∈{1,2,…,N} and l′∈{N+1,N+2,…,2N}. By Definition 1.1, it suffices to study the following system:
{(q2N+1−qk′)m2N+1mk′|q2N+1−qk′|3+∑N+1≤l≤2N(ql−qk′)mlmk′|ql−qk′|3+∑1≤k≠k′≤N(qk−qk′)mkmk′|qk−qk′|3=−λmk′(qk′−x0),(q2N+1−ql′)m2N+1ml′|q2N+1−ql′|3+∑1≤k≤N(qk−ql′)mkml′|qk−ql′|3+∑N+1≤l≠l′≤2N(ql−ql′)mlml′|qk−ql′|3=−λml′(ql′−x0),∑1≤k≤N(qk−q2N+1)mkm2N+1|qk−q2N+1|3+∑N+1≤l≤2N(ql−q2N+1)mlm2N+1|ql−q2N+1|3=−λm2N+1(q2N+1−x0). | (3.1) |
Thanks to (1.2), (1.3), and (3.1), the 2N+1 masses form a central configuration if and only if
{(a1eiα−eiθk′,h2N+1)m2N+1mk′[|a1eiα−eiθk′|2+h22N+1]32+∑N+1≤l≤2N(aei(θl+θ)−eiθk′,h)mlmk′[|aei(θl+θ)−eiθk′|2+h2]32+∑1≤k≠k′≤N(eiθk−eiθk′,0)mkmk′|eiθk−eiθk′|3=−λmk′(eiθk′−c0,−h0),(a1eiα−aei(θl′+θ),h2N+1−h)m2N+1ml′[|a1eiα−aei(θl′+θ)|2+(h2N+1−h)2]32+∑1≤k≤N(eiθk−aei(θl′+θ),−h)mkml′[|eiθk−aei(θl′+θ)|2+h2]32+∑N+1≤l≠l′≤2N(aei(θl+θ)−aei(θl′+θ),0)mlml′|aei(θl+θ)−aei(θl′+θ)|3 =−λml′(aei(θl′+θ)−c0,h−h0),∑1≤k≤N(eiθk−a1eiα,−h2N+1)mkm2N+1[|eiθk−a1eiα|2+h22N+1]32+∑N+1≤l≤2N(aei(θl+θ)−a1eiα,h−h2N+1)mlm2N+1[|aei(θl+θ)−a1eiα|2+(h−h2N+1)2]32=−λm2N+1(a1eiα−c0,h2N+1−h0). | (3.2) |
By the assumption that the values of N masses located at the vertices of one regular N-polygon are equal to each other, without loss of generality, we suppose that m1=m2=…=mN:=m>0, and we divide the proof of the necessity into four steps.
Step 1. We prove that a1=0.
Employing m1=m2=…=mN=m>0 and the second equation of (3.2), we have
(h2N+1−h)m2N+1[|a1eiα−aei(θl′+θ)|2+(h2N+1−h)2]32+∑1≤k≤N−hm[|eiθk−aei(θl′+θ)|2+h2]32=−λ(h−h0), | (3.3) |
where l′∈{N+1,N+2,…,2N}. Combining Lemma 2.4, (3.3),
h0=∑N+1≤l≤2Nmlh+m2N+1h2N+1m2N+1+∑1≤k≤Nmk+∑N+1≤l≤2Nml, |
and that λ is independent of the choice of l′, we deduce that
(h2N+1−h)m2N+1[|a1eiα−aei(θl′+θ)|2+(h2N+1−h)2]32≡constant, ∀l′∈{N+1,N+2,…,2N}. |
Thus, for any l′∈{N+1,N+2,…,2N}, we have |a1eiα−aei(θl′+θ)|2≡constant, i.e.,
|[a1cosα−acos(θl′+θ)]+i[a1sinα−asin(θl′+θ)]|2≡constant, ∀l′∈{N+1,N+2,…,2N}. |
Then, one computes that
a1acos(θl′+θ−α)≡constant, ∀l′∈{N+1,N+2,…,2N}. |
Since a represents the ratio of the sizes of the two regular N-polygons, a>0. Hence, if a1≠0, then
cos(θl′+θ−α)≡constant, ∀l′∈{N+1,N+2,…,2N}. | (3.4) |
In what follows, we assume that a1≠0, and we divide the proof of impossibility of a1≠0 into two cases: N=2 and N≥3.
(i) N=2:
In this case, l′∈{3,4}, and a1≠0. Then, by (3.4), we have cos(3π+θ−α)=cos(4π+θ−α), which implies that cos(θ−α)=0.
Under the assumption that m1=m2=m, we convert (1.2) and (1.3) into
{q1=(−1, 0), q2=(1, 0),q3=(aρ3eiθ, h)=(−aeiθ, h), q4=(aρ4eiθ, h)=(aeiθ, h), 0≤θ<2π, a>0, h≥0,q5=(a1eiα, h5), a1≥0, 0≤α≤2π, −∞<h5<+∞,c0=aeiθ(m4−m3)+a1m5eiαm1+m2+m3+m4+m5=aeiθ(m4−m3)+a1m5eiα2m+m3+m4+m5,h0=(m3+m4)h+m5h5m1+m2+m3+m4+m5=(m3+m4)h+m5h52m+m3+m4+m5. | (3.5) |
First, in (2.3), for any θ∈[0,2π) and any l′∈{N+1,N+2,…,2N} with N≥2, if we let μ=k, ˆμ=l′−N, ˜μ=0, κ=0, and ˜α=−θ∈(−2π,0], then the mapping
{{1,2,…,N}σ⟶{−θ,2πN−θ,…,2(N−1)πN−θ},σ(k)=[(k−(l′−N))(modN)]2πN−θ=[(k−l′)(modN)]2πN−θ, ∀k∈{1,2,…,N} |
is a bijection. Then, by the second equation of (3.2) and m1=m2=…=mN=m>0, we have
(1−a1aei(α−θ−θl′))m2N+1[|a−a1ei(α−θ−θl′)|2+(h2N+1−h)2]32+∑1≤k≤N(1−ei(θk−l′−θ)a)m[|a−ei(θk−l′−θ)|+h2]32+∑N+1≤l≠l′≤2N(1−eiθl−l′)mla3|1−eiθl−l′|3=(1−a1aei(α−θ−θl′))m2N+1[|a−a1ei(α−θ−θl′)|2+(h2N+1−h)2]32+∑1≤k≤N(1−ei(θk−θ)a)m[|a−ei(θk−θ)|2+h2]32+∑N+1≤l≠l′≤2N(1−eiθl−l′)mla3|1−eiθl−l′|3=λ−λac0e−i(θl′+θ), where l′∈{N+1,N+2,…,2N}. | (3.6) |
Note that all of the sides in the two regular N-polygons have the same size, and a represents the ratio of the sizes of the two regular N-polygons, so a=1. Choosing θl′=4π with l′=4, and then employing (3.6) with a=1 and N=2, we have
(1−a1ei(α−θ))m5[|1−a1ei(α−θ)|2+(h5−h)2]32+∑1≤k≤2(1−ei(θk−θ))m[|1−ei(θk−θ)|+h2]32+∑3≤l≠l′≤4(1−eiθl−l′)ml|1−eiθl−l′|3=λ−λc0e−iθ, where l′∈{3,4}. | (3.7) |
Combining N=2, (3.7) and the definition of circulant matrix C in (2.14),
CM=˜b1˜ξ1−˜b2˜ξ2, |
where
˜b1=λ−(1−a1ei(α−θ))m5[|1−a1ei(α−θ)|2+(h5−h)2]32−∑1≤k≤2(1−ei(θk−θ))m[|1−ei(θk−θ)|+h2]32, |
˜b2=λe−iθc0=λe−iθaeiθ(m4−m3)+a1m2N+1eiαm1+m2+m3+m4+m5, |
M=(m3,m4)T, ˜ξ1=(1,1)T, and ˜ξ2=(ρ1,ρ21)T=(−1,1)T. Thus, we have
(˜b1−˜b2ρ1˜b1−˜b2ρ2)=(01−ρ−1|1−ρ−1|31−ρ1|1−ρ1|30)(m3m4). |
Then, by ρ−1=ρ−1+2=ρ1=−1, one computes that ˜b2=(m4−m3)/8.
On the other hand, by (3.5), we have
m4−m38=˜b2=λe−iθc0=λa(m4−m3)+a1m2N+1ei(α−θ)2m+m3+m4+m5∈R. |
In addition, according to lines 1-8 of page 109 of [7], we have λ>0 for Definition 1.1. Combining a∈R and a1≠0, one computes that Im(ei(α−θ))=0, i.e., sin(α−θ)=0, which contradicts with cos(θ−α)=0. Thus, cos(θ−α)=0 is impossible, which implies that for the spatial twisted central configuration with N=2, we deduce the conclusion that a1=0.
(ii) N≥3:
For (3.4), if we let β=θ−α and choose l′=N+1,N+2, and N+3, then
{cos(4πN+β)−cos(2πN+β)=0,cos(6πN+β)−cos(2πN+β)=0, |
and this is equivalent to
{sinπNsin(β+3πN)=0,sin2πNsin(β+4πN)=0. | (3.8) |
Observing that N≥3, sin(π/N)≠0, and sin(2π/N)≠0. Combining with (3.8), we have
{β=k1π−3πN, k1∈Z,β=k2π−4πN, k2∈Z, |
which implies that k1π−3π/N=k2π−4π/N. So, (k2−k1)π=π/N where positive integer N≥3, and this is impossible. Therefore, (3.4) does not hold. Then, for the spatial twisted central configuration with N≥3, we arrive at the conclusion that a1=0, too.
Step 2. We prove that θ=sπ/N with s∈{0,1,…,2N−1}, and we divide the proof into two sub-steps.
Step 2.1. We show that mN+1=mN+2=…=m2N.
In fact, inserting a=1 and a1=0 into (3.6), we have
m2N+1[1+(h2N+1−h)2]32+∑1≤k≤N(1−ei(θk−θ))m[|1−ei(θk−θ)|2+h2]32+∑N+1≤l≠l′≤2N(1−eiθl−l′)ml|1−eiθl−l′|3=λ−λc0e−i(θl′+θ), where l′∈{N+1,N+2,…,2N}. | (3.9) |
Combining N≥2, m1=m2=…=mN=m, (3.9), ρd=eiθd with θd=2dπ/N and d∈Z, along with the definition of circulant matrix C in (2.14),
CM=b1ξ1−b2ξN, | (3.10) |
where
b1=λ−m2N+1[1+(h2N+1−h)2]32−∑1≤k≤N(1−ei(θk−θ))m[|1−ei(θk−θ)|+h2]32, |
b2=λe−iθc0=λe−iθ∑1≤k≤Nmkρk+∑N+1≤l≤2Nmlρleiθm2N+1+∑1≤k≤Nmk+∑N+1≤l≤2Nml=λ∑N+1≤l≤2Nmlρlm2N+1+∑1≤k≤Nmk+∑N+1≤l≤2Nml, | (3.11) |
M=(mN+1,mN+2,…,m2N)T, ξ1=(1,1,…,1)T and ξN=(ρN−1,ρ2N−1,…,ρNN−1)T.
In the following, we divide the proof of mN+1=mN+2=…=m2N into three cases: N=2, N=3, and N≥4.
(i) N=2:
By (3.10) with N=2,
(b1−b2ρ1b1−b2ρ2)=(01−ρ−1|1−ρ−1|31−ρ1|1−ρ1|30)(m3m4). |
Moreover, when N=2, it is easy to see that ρ1+ρ2=0 and ρ−1=ρ1=−1. Thus,
2b2=1−ρ−1|1−ρ−1|3m4−1−ρ1|1−ρ1|3m3=1−ρ1|1−ρ1|3(m4−m3), |
which implies that b2=(m4−m3)/8. Thus, when N=2, then inserting ρ3=−1, and ρ4=1 into (3.11) and combining with b2=(m4−m3)/8, we have
b2=λ(m4−m3)∑1≤k≤5mk=(m4−m3)8. | (3.12) |
In what follows, we prove that for the spatial twisted central configuration with N=2, m3=m4, and we prove it by contradiction. We assume m3≠m4.
In fact, on the one hand, by m3≠m4, m1=m2=m, and (3.12), we have
λ=(2m+m3+m4+m5)8. | (3.13) |
Moreover, thanks to N=2, a=1, a1=0, and the fourth equation of (3.5), one computes that
c0=aeiθ(m4−m3)+a1m5eiα2m+m3+m4+m5=eiθ(m4−m3)2m+m3+m4+m5. | (3.14) |
Summing the equations of the first part of (3.2) over k′=1 and k′=2, by N=2, a1=0, m1=m2=m, (3.13), and (3.14), we have
∑1≤k′≤2−eiθk′m5[1+h25]32+∑1≤k′≤2∑3≤l≤4(aei(θl+θ)−eiθk′)ml[|aei(θl+θ)−eiθk′|2+h2]32+∑1≤k′≤2∑1≤k≠k′≤2(eiθk−eiθk′)mk|eiθk−eiθk′|3=−λ∑1≤k′≤2eiθk′+2λc0=eiθ(m4−m3)4. |
Then, combining eiθ1=eiθ−1=−1 (N=2), we have
0+∑1≤k′≤2eiθk′∑3≤l≤4(aei(θl−k′+θ)−1)ml[|aei(θl−k′+θ)−1|2+h2]32+∑1≤k′≤2∑1≤k≠k′≤2(eiθk−eiθk′)m|eiθk−eiθk′|3=∑1≤k′≤2eiθk′∑3≤l≤4(aei(θl+θ)−1)ml[|aei(θl+θ)−1|2+h2]32+∑1≤k′≤2eiθk′∑1≤k≠k′≤2(eiθk−k′−1)m|eiθk−k′−1|3=(eiθ1−1)m|eiθ1−1|3∑1≤k′≤2eiθk′=0=eiθ(m4−m3)4, |
i.e., m3=m4, and this contradicts with the assumption that m3≠m4. Hence, for the spatial twisted central configuration with N=2, we deduce that m3=m4.
(ii) N=3:
By (3.10),
(b1−b2ρ2b1−b2ρ1b1−b2ρ3)=(01−ρ−1|1−ρ−1|31−ρ−2|1−ρ−2|31−ρ1|1−ρ1|301−ρ2|1−ρ2|31−ρ2|1−ρ2|31−ρ1|1−ρ1|30)(m4m5m6). | (3.15) |
From N=3, we have ρ−1=ρ2 and ρ−2=ρ1; thus, ρ1+ρ2+ρ3=0. Together with (3.15) and
{Re(1−ρ1|1−ρ1|3)=Re(1−ρ2|1−ρ2|3),Im(1−ρ1|1−ρ1|3)=−Im(1−ρ2|1−ρ2|3), |
there is
3b1=3b1−b2(ρ1+ρ2+ρ3)=1−ρ−1|1−ρ−1|3m5+1−ρ−2|1−ρ−2|3m6+1−ρ1|1−ρ1|3m4+1−ρ2|1−ρ2|3m6+1−ρ2|1−ρ2|3m4+1−ρ1|1−ρ1|3m5=(1−ρ1|1−ρ1|3+1−ρ2|1−ρ2|3)m4+(1−ρ1|1−ρ1|3+1−ρ2|1−ρ2|3)m5+(1−ρ1|1−ρ1|3+1−ρ2|1−ρ2|3)m6∈R, |
which implies that b1∈R.
On the other hand, for N=3, Lemma 2.9 gives us information that there exist constants c1, c2, and c3 such that M=c1ξ1+c2ξ2+c3ξ3 where M=(m4,m5,m6)T. Thus, combining (3.10), we obtain
c1λ1(C)ξ1+c2λ2(C)ξ2+c3λ3(C)ξ3=b1ξ1−b2ξ3. | (3.16) |
Then, it follows from (3.16) and Lemma 2.9 that c1λ1(C)ξ1=b1ξ1 and c3λ3(C)ξ3=−b2ξ3.
Employing Lemma 2.5, Lemma 2.7, ρ3=1, ρ4=ρ1, ρ1+ρ2=−1, and |1−ρ1|=|1−ρ2|, we have
{λ1(C)=1−ρ1|1−ρ1|3+1−ρ2|1−ρ2|3=∑1≤j≤21−eiθj|1−eiθj|3∈R,λ3(C)=(1−ρ1)ρ2|1−ρ1|3+(1−ρ2)ρ1|1−ρ2|3=ρ2−ρ3+ρ1−ρ3|1−ρ1|3=−3|1−ρ1|3∈R. | (3.17) |
Then, thanks to b1∈R, ξ1=(1,1,…,1)T, and c1λ1(C)ξ1=b1ξ1, one computes that c1∈R. In what follows, we will prove that c2∈R and c3∈R.
By ξ1=(ρ0,ρ0,ρ0), ξ2=(ρ1,ρ2,ρ3), ξ3=(ρ2,ρ1,ρ3), and M=c1ξ1+c2ξ2+c3ξ3 with N=3, we have
{Im(c1ρ0+c2ρ1+c3ρ2)=0,Im(c1ρ0+c2ρ2+c3ρ1)=0,Im(c1ρ0+c2ρ3+c3ρ3)=0. | (3.18) |
Based upon ρ0=ρ3=1, c1∈R, and the third equation of (3.18), we have c2+c3∈R.
Employing ρ0=1, c1∈R, and the first equation of (3.18), we see that c2ρ1+c3ρ2∈R. Note that
c2ρ1+c3ρ2=(c2cos2π3+ic2sin2π3)+(c3cos4π3+ic3sin4π3)=(c2+c3)cos2π3+(c2−c3)isin2π3, |
and then c2=c3. Therefore, with the help of c2+c3∈R, we have c3∈R.
In virtue of (3.17), λ3(C)∈R. Moreover, combining c3∈R and c3λ3(C)ξ3=−b2ξ3 where ξ3=(ρ2,ρ4,ρ6)T=(ρ2,ρ1,ρ3)T, one computes that b2∈R.
By now, for (3.15), by the accumulated facts b1∈R, b2∈R, |1−ρ1|=|1−ρ2|, ρ−1=ρ2, ρ−2=ρ1, and Im(ρ1)=−Im(ρ2), we have m4=m5=m6.
(iii) N≥4:
Lemma 2.9 gives us information that there exist constants ˜c1,˜c2,…,˜cN such that M=˜c1ξ1+˜c2ξ2+…+˜cNξN where M=(mN+1,mN+2,…,m2N)T. We can regard (3.10) as (3.11) of [26]; moreover, we regard C, b1, and b2 of this paper as Aα, ∑Nk=1mk, and ∑Nk=1mkqk of [26], respectively. Then, combining N≥4 and Lemmas 2.8–2.10, similar to the procedure of Case 2.1 on pages 6–7 of [26], we obtain mN+1=mN+2=…=m2N.
Step 2.2. Based on Step 1 and Step 2.1, we prove that θ=sπ/N with s∈{0,1,…,2N−1}.
Inserting m1=m2=…=mN, a1=0, and mN+1=mN+2=…=m2N into the second equality of (1.3), we have c0=0. Then, with the help of the first equation of (3.2), for any k′∈{1,2,…,N}, we obtain
−m2N+1[1+h22N+1]32+∑N+1≤l≤2N(aei(θl−k′+θ)−1)ml[|aei(θl−k′+θ)−1|2+h2]32+∑1≤k≠k′≤N(eiθk−k′−1)mk|eiθk−k′−1|3=−λ∈R. | (3.19) |
For any k′∈{1,2,…,N}, it follows from Remark 2.2 that the mapping
{1,2,…,N}∖{k′}σ2⟶{2πN,4πN,…,2(N−1)πN}, |
where
σ2(k)=[(k−k′)(modN)]2πN, ∀k∈{1,2,…,N}∖{k′}, |
is a bijection. Thus, by the procedure of obtaining (2.6), and θd=2πd/N with d∈Z, we have
{∑k≠k′1≤k≤Nsinθk−k′|2−2cosθk−k′|3=∑1≤j≤N−1sinθj|2−2cosθj|3=0, ∀k′∈{1,2,…,N},∑k≠k′1≤k≤Ncosθk−k′−1|2−2cosθk−k′|3=∑1≤j≤N−1cosθj−1|2−2cosθj|3, ∀k′∈{1,2,…,N},∑k≠k′1≤k≤N1|2−2cosθk−k′|3=∑1≤j≤N−11|2−2cosθj|3, ∀k′∈{1,2,…,N}, | (3.20) |
and then
∑1≤k≠k′≤Neiθk−k′−1|eiθk−k′−1|3∈R. | (3.21) |
Combining (3.19) with (3.21), we have
Im(∑N+1≤l≤2Naei(θl−k′+θ)−1[|aei(θl−k′+θ)−1|2+h2]32)=0, where h>0 and k′∈{1,2,…,N}. | (3.22) |
For any l∈{N+1,N+2,…,2N}, in (2.5), if we let ˜s=0, then the mapping
{{1,2,…,N}σ1⟶{θ,2πN+θ,…,2(N−1)πN+θ},σ1(k′)=[(l−k′)(modN)]2πN+θ, ∀k′∈{1,2,…,N} |
is a bijection, too. Hence, we have
{∑1≤k′≤Nacos(θl−k′+θ)−1[1+a2−2acosθ(l−k′+πN)+h2]32=∑1≤j≤Nacos(θj+θ)−1[1+a2−2acos(θj+θ)+h2]32,∑1≤k′≤Nasin(θl−k′+θ)[1+a2−2acos(θl−k′+θ)+h2]32=∑1≤j≤Nasin(θj+θ)[1+a2−2acos(θj+θ)+h2]32. | (3.23) |
Using the definition of f(θ) in (2.1), (3.22), and the second equation of (3.23), we can see that
f(θ)=∑1≤j≤Nasin(θj+θ)[|1+a2−2acos(θj+θ)|2+h2]32=0. | (3.24) |
On the one hand, if θ∈(2˜sπ/N,2˜sπ/N+π/N) where ˜s∈{0,1,…,N−1}, then by Lemmas 2.1-2.2, there is f(θ)>0, which contradicts (3.24).
On the other hand, if θ∈(2˜sπ/N+π/N,2˜sπ/N+2π/N) where ˜s∈{0,1,…,N−1}, then by Lemma 2.1, we have f(θ)=−f(−θ)=−f(−θ+2π/N) and −θ+2π/N∈(−2˜sπ/N,−2˜sπ/N+π/N). Therefore, it follows from Lemma 2.2 that f(θ)<0, which also contradicts (3.24).
Thus, combining θ∈[0,2π), we conclude that the twist angle must be θ=2˜sπ/N or θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}, and then θ=sπ/N with s∈{0,1,…,2N−1}.
Step 3. We show that m1=m2=…=mN=mN+1=mN+2=…=m2N.
Based on the first part of Step 2, we can assume that mN+1=mN+2=…=m2N:=bm where constant b>0. By the assumption that m1=m2=…=mN:=m, it suffices to show that the value of b can only take b=1, and we prove it by contradiction. We assume that b≠1.
Thanks to a=1, m1=m2=…=mN:=m, a1=0, and mN+1=mN+2=…=m2N:=bm, for (3.2) we have
{Nm2N+1h2N+1[1+h22N+1]32+∑N+1≤l≤2Nbmh[|ei(θl−k′+θ)−1|2+h2]32=λh0,Nm2N+1(h2N+1−h)[1+(h2N+1−h)2]32−∑1≤k≤Nmh[|ei(θk−l′−θ)−1|2+h2]32=λ(h0−h),Nmh2N+1[1+h22N+1]32+Nbm(h2N+1−h)[1+(h−h2N+1)2]32=λ(h2N+1−h0), | (3.25) |
where k′∈{1,2,…,N} and l′∈{N+1,N+2,…,2N}. Combining the first and second equations of (3.25) with (2.7),
{Nm2N+1h2N+1[1+h22N+1]32+∑1≤j≤Nbmh[|ei(θj+θ)−1|2+h2]32=λh0,Nm2N+1(h2N+1−h)[1+(h2N+1−h)2]32−∑1≤j≤Nmh[|ei(θj+θ)−1|2+h2]32=λ(h0−h),Nmh2N+1[1+h22N+1]32+Nbm(h2N+1−h)[1+(h2N+1−h)2]32=λ(h2N+1−h0). | (3.26) |
Let
{ˆx=Nh2N+1[1+h22N+1]32,y=N(h2N+1−h)[1+(h2N+1−h)2]32,z=∑1≤j≤Nmh[|ei(θj+θ)−1|2+h2]32. | (3.27) |
Thus, (3.26) can be simplified into
{m2N+1ˆx+bz=λh0,m2N+1y−z=λ(h0−h),mˆx+bmy=λ(h2N+1−h0). | (3.28) |
On the one hand, by (3.28), we see that
{m2N+1ˆx−m2N+1y+(b+1)z=λh,mˆx+bmy=λ(h2N+1−h0), | (3.29) |
and
{m2N+1ˆx+m2N+1y+(b−1)z=2λh0−λh,mˆx+bmy=λ(h2N+1−h0). | (3.30) |
Then, it follows from (3.29), (3.30), and b≠1 that
−λh0m2N+1+λm2N+1h2N+1−λmh+m(b+1)zmm2N+1(b+1)=y=−λh0m2N+1−2λmh0+λm2N+1h2N+1+λmh+m(b−1)zmm2N+1(b−1). |
Thus,
h0=bmh+m2N+1h2N+1m2N+1+m+bm. |
Combining with
h0=∑N+1≤l≤2Nmlh+m2N+1h2N+1m2N+1+∑1≤k≤Nmk+∑N+1≤l≤2Nml=bNmh+m2N+1h2N+1m2N+1+Nm+bNm, | (3.31) |
we have
bmh+m2N+1h2N+1m2N+1+m+bm=bNmh+m2N+1h2N+1m2N+1+Nm+bNm, |
which implies that (1+b)h2N+1=bh. Moreover, with the help of (3.31), one computes that h0=h2N+1. Then, it follows from the second equation of (3.29) that x=−by. Hence, with the aid of (3.27) and (1+b)h2N+1=bh, we have
bh1+b[1+b2h2(1+b)2]32=bh1+b[1+h2(1+b)2]32, |
that is, b=±1, which contradicts b>0 and the assumption b≠1. So, b=1, and we arrive at the conclusion that m1=m2=…=mN=mN+1=mN+2=…=m2N.
Step 4. We prove that h2N+1=h/2.
In virtue of (3.30), we have
m(2λh0−λh)=m2N+1λ(h2N+1−h0). |
Note that lines 1–8 of page 109 of [7] show us that in Definition 1.1, λ must be a positive number. Then, combining the last equality of (1.3) and m1=m2=…=mN=mN+1=mN+2=…=m2N=m,
m(2Nmh+2h2N+1m2N+12Nm+m2N+1−h)=m2N+1(−Nmh+h2N+1m2N+12Nm+m2N+1+h2N+1), |
which implies that N=1 or h=2h2N+1. Combining with N≥2, for the spatial twisted central configuration, we have h2N+1=h/2.
Remark 3.1. The proof of Step 1 is independent of the condition that a=1. That is, for the twisted central configuration of the (2N+1)-body problem with the assumption that m1=m2=…=mN, and without the assumption that a=1, we have a1=0. That is, the (2N+1)-th mass must be in the vertical line of the two paralleled planes containing the two regular N-polygons, respectively, and the vertical line segment passes through the geometric centers of the two regular N-polygons.
We divide the proof into two steps.
Step 1. Based on the assumptions that θ=sπ/N with s∈{0,1,…,2N−1}, a=1, h2N+1=h/2, a1=0, and m1=m2=…=mN=mN+1=mN+2=…=m2N=m, we show that if there exists a constant λ∈R such that
{m2N+1[1+h24]32+∑1≤j≤N(1−ei(θj+θ))m[|ei(θj+θ)−1|2+h2]32+∑1≤j≤N(1−eiθj)m|eiθj−1|3=λ,m2N+1[1+h24]32+∑1≤j≤N2m[|ei(θj+θ)−1|2+h2]32=λ, | (3.32) |
then the 2N+1 masses can form a central configuration.
In fact, by (1.3), in this situation we get
{c0=∑1≤k≤Nmρk+meiθ∑N+1≤l≤2Nρl2Nm+m2N+1=0,h0=∑N+1≤l≤2Nmh+m2N+1h2N+12Nm+m2N+1=Nmh+m2N+1h22Nm+m2N+1=h2. | (3.33) |
Employing a=1, h2N+1=h/2, a1=0, m1=m2=…=mN=mN+1=mN+2=…=m2N=m, (3.33) and
∑1≤k≤Neiθk=∑N+1≤l≤2Nei(θl+θ)=0, |
we see that (3.2) holds if and only if
{(−eiθk′,h2)mm2N+1[1+h24]32+∑N+1≤l≤2N(ei(θl+θ)−eiθk′,h)m2[|ei(θl+θ)−eiθk′|2+h2]32+∑1≤k≠k′≤N(eiθk−eiθk′,0)m2|eiθk−eiθk′|3=−λm(eiθk′,−h2),(−ei(θl′+θ),−h2)mm2N+1[1+h24]32+∑1≤k≤N(eiθk−ei(θl′+θ),−h)m2[|eiθk−ei(θl′+θ)|2+h2]32+∑N+1≤l≠l′≤2N(ei(θl+θ)−ei(θl′+θ),0)m2|ei(θl+θ)−ei(θl′+θ)|3=−λm(ei(θl′+θ),h2). | (3.34) |
Therefore, it suffices to verify (3.34) holds. Clearly, (3.34) is equivalent to
{(−1,h2)m2N+1[1+h24]32+∑N+1≤l≤2N(ei(θl−k′+θ)−1,h)m[|ei(θl−k′+θ)−1|2+h2]32+∑1≤k≠k′≤N(eiθk−k′−1,0)m|eiθk−k′−1|3=−λ(1,−h2),(−1,−h2)m2N+1[1+h24]32+∑1≤k≤N(ei(θk−l′−θ)−1,−h)m[|ei(θk−l′−θ)−1|2+h2]32+∑N+1≤l≠l′≤2N(eiθl−l′−1,0)m|eiθl−l′−1|3=−λ(1,h2). | (3.35) |
On the other hand, for any l′∈{N+1,N+2,…,2N}, it follows from Remark 2.2 that the mapping
{N+1,N+2,…,2N}σ3⟶{2πN,4πN,…,2(N−1)πN}, |
where
σ3(l)=[(l−l′)(modN)]2πN, ∀l∈{N+1,N+2,…,2N}∖{l′}, |
is a bijection. Thus, by θd=2πd/N with d∈Z, we have
{∑l≠l′N+1≤l≤2Nsinθl−l′|2−2cosθl−l′|3=∑1≤j≤N−1sinθj|2−2cosθj|3, ∀l′∈{N+1,N+2,…,2N},∑l≠l′N+1≤l≤2Ncosθl−l′−1[|2−2cosθl−l′|3=∑1≤j≤N−1cosθj−1|2−2cosθj|3, ∀l′∈{N+1,N+2,…,2N},∑l≠l′N+1≤l≤2N1|2−2cosθl−l′|3=∑1≤j≤N−11|2−2cosθj|3, ∀l′∈{N+1,N+2,…,2N}. |
Together with (3.20), for any k′∈{1,2,…,N} and any l′∈{N+1,N+2,…,2N}, it follows that
∑k≠k′1≤k≤N(eiθk−k′−1)m|eiθk−k′−1|3=−∑1≤j≤N−1(1−eiθj)m|eiθj−1|3=∑l≠l′N+1≤l≤2N(eiθl−l′−1)m|eiθl−l′−1|3. | (3.36) |
Employing Lemma 2.3, (3.35), and (3.36), we conclude that if there exists a constant λ∈R such that (3.32) holds, then by Definition 1.1, the 2N+1 masses form a central configuration.
Step 2. We prove the existence of the spatial twisted central configuration, i.e., we prove the existence of λ of Step 1.
Define the function g as follows:
g(h)=−12∑1≤j≤N(1+ei(θj+θ))m[|ei(θj+θ)−1|2+h2]32+12∑1≤j≤N−1(1−eiθj)m|eiθj−1|3, | (3.37) |
where h>0 and θ=sπ/N with s∈{0,1,…,2N−1}. Thanks to Lemmas 2.3, 2.5, and (3.37), we see that g(h)∈R, which implies that
g(h)=−12∑1≤j≤N(1+cos(θj+θ))m[2+h2−2cos(θj+θ)]32+12∑1≤j≤N−1(1−cosθj)m|2−2cosθj|32. | (3.38) |
In what follows, we prove that there exists h=ˉh(N) such that g(ˉh(N))=0. Note that θ=sπ/N with s∈{0,1,…,2N−1} is equivalent to θ=2˜sπ/N or θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}. We divide the following proof into two cases.
Case 1. θ=2˜sπ/N with ˜s∈{0,1,…,N−1}.
On the one hand, since m>0, θ=2˜sπ/N, and 1+cosθj≥0 with j∈Z, we have
−12∑1≤j≤N(1+cos(θj+θ))m[2+h2−2cos(θj+θ)]32=−12∑1+˜s≤j+˜s≤N+s(1+cosθj+˜s)m[2+h2−2cosθj+˜s]32=−12∑1≤j≤N(1+cosθj)m[2+h2−2cosθj]32<0. | (3.39) |
Moreover, if h→0+, then
−12∑1≤j≤N(1+cosθj)m[2+h2−2cosθj]32→−∞, |
which implies that when the twist angle is θ=2˜sπ/N with ˜s∈{0,1,…,N−1}, there exists h=h1(N) such that g(h1(N))<0.
On the other hand, notice that if h→+∞, then
−12∑1≤j≤N(1+cosθj)m[2+h2−2cosθj]32→0. |
Then, by (3.38)–(3.39), we see that when the twist angle is θ=2˜sπ/N with ˜s∈{0,1,…,N−1}, there exists h=h2(N) such that g(h2(N))>0.
Hence for the case of θ=2˜sπ/N with ˜s∈{0,1,…,N−1}, employing the fact that g is a continuous function, there exists h=ˉh(N) such that g(ˉh(N))=0.
Case 2. θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}.
In this case, by (3.38), ˜s∈{0,1,…,N−1}, and θd=2πd/N with d∈Z, we have
g(h)=m2[−∑1≤j+˜s≤N+˜s1+cos(θj+˜s+πN)[2+h2−2cos(θj+˜s+πN)]32+∑1≤j≤N−11−cosθj|2−2cosθj|32]=m2[−∑1≤j≤N1+cos(θj+πN)[2+h2−2cos(θj+πN)]32+∑1≤j≤N−11−cosθj|2−2cosθj|32]. | (3.40) |
Then, it follows from (3.40) and Lemma 2.5 that
g(h)=m2[−∑1≤j≤N1+cos(θj+πN)[2+h2−2cos(θj+πN)]32+∑1≤j≤N−11−eiθj|1−eiθj|32]. | (3.41) |
If h→0, then with the help of (3.41) and Lemma 2.6, limh→0g(h)<0, which implies that when the twist angle is θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}, there exists h=h3(N) such that g(h3(N))<0.
On the other hand, if h→+∞, then
−∑1≤j≤N1+cos(θj+πN)[2+h2−2cos(θj+πN)]32→0, |
which implies that when the twist angle is θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}, there exists h=h4(N) such that g(h4(N))>0.
Therefore, for the case of θ=2˜sπ/N+π/N with ˜s∈{0,1,…,N−1}, combining with the continuity of function g, there exists h=ˉh(N) such that g(ˉh(N))=0.
By now, Cases 1–2 show us that when θ=sπ/N with s∈{0,1,…,2N−1}, there exists h=ˉh(N) such that g(ˉh(N))=0, which implies that
12∑1≤j≤N(1−ei(θj+θ))m[|ei(θj+θ)−1|2+(ˉh(N))2]32+12∑1≤j≤N−1(1−eiθj)m|eiθj−1|3=∑1≤j≤Nm[|ei(θj+θ)−1|2+(ˉh(N))2]32. |
Moreover, note that
∑1≤j≤Nm[|ei(θj+θ)−1|2+(ˉh(N))2]32∈R. |
Then, there is a constant λ∈R such that
12∑1≤j≤N(1−ei(θj+θ))m[|ei(θj+θ)−1|2+(ˉh(N))2]32+12∑1≤j≤N(1−eiθj)m|eiθj−1|3=12λ−12m2N+1[1+(ˉh(N))24]32=∑1≤j≤Nm[|ei(θj+θ)−1|2+(ˉh(N))2]32>0, |
i.e.,
{12m2N+1[1+(ˉh(N))24]32+12∑1≤j≤N(1−ei(θj+θ))m[|ei(θj+θ)−1|2+(ˉh(N))2]32+12∑1≤j≤N(1−eiθj)m|eiθj−1|3=12λ,12m2N+1[1+(ˉh(N))24]32+∑1≤j≤Nm[|ei(θj+θ)−1|2+(ˉh(N))2]32=12λ, |
which implies that there exists a constant λ∈R such that (3.32) holds. Then, by Step 1, the 2N+1 masses form a central configuration.
For the spatial twisted central configuration, (3.2) holds. Moreover, note that m1=m2=…=mN=m and a=1. Then, all the assumptions of Theorem 1.1 are satisfied, so we have a1=0, h2N+1=h/2, and m1=m2=…=mN=mN+1=…=m2N=m. Thus, c0=0, h0=h/2, and q2N+1=(0+0i,h/2). Thus, in the following, it suffices to prove the uniqueness of h.
In fact, by (3.36) and the first equation of (3.2), for any k′∈{1,2,…,N} and any l′∈{N+1,N+2,…,2N}, one computes that
{m2N+1[1+h24]32h2+∑N+1≤l≤2N[(1−ei(θl−k′+θ))m][|1−ei(θl−k′+θ)|2+h2]32h2+∑1≤j≤N−1[(1−eiθj)m]|1−eiθj|3h2=λh2,h2m2N+1[1+h24]32+∑1≤j≤Nmh[|1−ei(θj+θ)|2+h2]32=λh0=λh2, |
where h>0 and θ∈[0,2π). Hence, it follows from Lemma 2.3 that
{m2N+1h2[1+h24]32+∑1≤j≤N(1−ei(θj+θ))mh2[|1−ei(θj+θ)|2+h2]32+∑1≤j≤N−1(1−eiθj)mh2|1−eiθj|3=λh2,h2m2N+1[1+h24]32+∑1≤j≤Nmh[|1−ei(θj+θ)|2+h2]32=λh2. | (4.1) |
Let
{ˉx=∑1≤j≤N−11−eiθj|1−eiθj|3,ˉy(h)=∑1≤j≤Ncos(θj+θ)[2−2cos(θj+θ)+h2]32,ˉz(h)=∑1≤j≤N1[2−2cos(θj+θ)+h2]32. | (4.2) |
Combining λ∈R, h∈R, (4.1), and (4.2), we have
{h2m2N+1[1+h24]32+mh2[ˉz(h)−ˉy(h)]+mh2ˉx=λh2,h2m2N+1[1+h24]32+mhˉz(h)=λh2, |
which implies that
mh2[ˉz(h)−ˉy(h)]−mhˉz(h)+mh2ˉx=−λh2, |
and then we obtain ˉx=ˉy(h)+ˉz(h). Hence, if the 2N+1 masses form a central configuration, then the distance h must satisfy that ˉx=ˉy(h)+ˉz(h). Next, we prove the uniqueness of the distance h.
We take G(h)=ˉy(h)+ˉz(h)−ˉx for h>0, and by (4.2), it is easy to verify that G′(h)<0. Moreover, note that m1=m2=…=mN=m and a=1. Then, employing Theorem 1.1, we obtain that the existence of the central configuration implies that θ=sπ/N with s∈{0,1,…,2N−1} holds. Then, due to the rotational symmetry of the central configuration, in order to prove Theorem 1.2, it suffices to consider the following two cases: θ=0 and θ=π/N.
Case 1. θ=0.
Lemma 2.5 shows that
ˉx=∑1≤j≤N−11−eiθj|1−eiθj|3∈R, |
and then by (4.2),
G(h)=ˉy(h)+ˉz(h)−ˉx=∑1≤j≤N1+cosθj[2−2cosθj+h2]32−∑1≤j≤N−11−cosθj[2−2cosθj]32=∑1≤j≤N−11+cosθj[2−2cosθj+h2]32+2h3−∑1≤j≤N−11−cosθj[2−2cosθj]32. | (4.3) |
Employing (4.3), one verifies that there exist a small enough constant h=ˉh>0 and a big enough constant h=˜h>0 such that G(ˉh)>0 and G(˜h)<0. Then, combining the continuity and monotonicity of function G, for the case of θ=0, there is a unique h=ˆh>0, such that G(ˆh)=0, i.e., ˉx=ˉy(ˆh)+ˉz(ˆh).
Case 2. θ=π/N.
Employing (4.2), we have
ˉy(h)+ˉz(h)=∑1≤j≤N1+cos(θj+θ)[2−2cos(θj+θ)+h2]32=∑1≤j≤N1+cos(θj+πN)[2−2cos(θj+πN)+h2]32=∑1≤j≤N1+cos(θj+πN)[2−2cos(θj+πN)+h2]32. |
By Lemma 2.6, limh→0G(h)>0. Furthermore, by Lemma 2.5, we see that limh→+∞G(h)<0. Thus, for the case of θ=π/N, due to the continuity and monotonicity of function G, there is a unique h=˘h>0, such that G(˘h)=0, i.e., ˉx=ˉy(˘h)+ˉz(˘h).
Based upon Cases 1–2, there exists only one h>0 such that ˉx=ˉy(h)+ˉz(h), i.e., there is only one h>0 such that the 2N+1 masses form a central configuration. Moreover, combining the other two conclusions that a1=0 and h2N+1=h/2, we obtain that q2N+1=(a1eiα,h2N+1) is unique, i.e., there are only one positive distance h between the two paralleled regular N-polygons and only one position q2N+1 of the (2N+1)-th mass such that the 2N+1 masses form a central configuration.
The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.
The authors sincerely thank the referees and the editors for their valuable comments and suggestions, and the authors also express their sincere gratitude to professor Shiqing Zhang for his helpful discussions and suggestions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 12361022 and 12371163), the China Postdoctoral Science Foundation (Grant No. 2021M700968), the Guizhou Provincial Basic Research Program (Natural Science, Grant Nos. ZK[2021]003 and ZK[2023]138), and the Natural Science Research Project of the Department of Education of Guizhou Province (Grant No. QJJ2022047).
The authors declare there is no conflict of interest.
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