Research article Special Issues

Decision models enhancing environmental flow sustainability: A strategic approach to water resource management

  • Received: 27 May 2024 Revised: 26 July 2024 Accepted: 10 October 2024 Published: 16 October 2024
  • Proper water resource management is essential for maintaining a sustainable supply chain and meeting water demand. The urgent need to preserve river ecosystems by sustaining environmental flow (EF) in the realm of environmental management has been highlighted by the drastic changes to river ecosystems and upstream flow dynamics brought about by careless river exploitation in the last few decades. To optimize EF in river basin management, we present an integrated modeling approach. We focused on the Pir Khezran River basin. Our objective was to estimate EF and generalize the findings to adjacent rivers using modeling techniques, thus providing valuable insights for environmental management applications. The assessment and optimization of EF under uncertain conditions was achieved by combining physical habitat simulation (PHABSIM) modeling with advanced techniques like Adaptive Neuro-Fuzzy Inference Systems (ANFIS) and Multilayer Perceptron (MLP) neural networks. This integrated modeling approach contributes to sustainable solutions for river basin management and environmental conservation by effectively optimizing EF, as demonstrated by the results. This research, therefore, makes valuable contributions to environmental management in various areas such as ecological preservation, modeling and optimizing environmental systems, and policy considerations.

    Citation: Seiran Haghgoo, Jamil Amanollahi, Barzan Bahrami Kamangar, Shahryar Sorooshian. Decision models enhancing environmental flow sustainability: A strategic approach to water resource management[J]. AIMS Environmental Science, 2024, 11(6): 900-917. doi: 10.3934/environsci.2024045

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  • Proper water resource management is essential for maintaining a sustainable supply chain and meeting water demand. The urgent need to preserve river ecosystems by sustaining environmental flow (EF) in the realm of environmental management has been highlighted by the drastic changes to river ecosystems and upstream flow dynamics brought about by careless river exploitation in the last few decades. To optimize EF in river basin management, we present an integrated modeling approach. We focused on the Pir Khezran River basin. Our objective was to estimate EF and generalize the findings to adjacent rivers using modeling techniques, thus providing valuable insights for environmental management applications. The assessment and optimization of EF under uncertain conditions was achieved by combining physical habitat simulation (PHABSIM) modeling with advanced techniques like Adaptive Neuro-Fuzzy Inference Systems (ANFIS) and Multilayer Perceptron (MLP) neural networks. This integrated modeling approach contributes to sustainable solutions for river basin management and environmental conservation by effectively optimizing EF, as demonstrated by the results. This research, therefore, makes valuable contributions to environmental management in various areas such as ecological preservation, modeling and optimizing environmental systems, and policy considerations.



    In fields such as data structures, computer algorithms, data encryption, security, and networking, computer science relies heavily on graph theory. For instance, designs of a database, routing problems, and networking based on the key ideas of graph theory, namely cycles and trees. Many computer security algorithms and ciphers are similarly based on modular arithmetic from number theory. In these areas, a strong mathematical background and a clear understanding of modular arithmetic, graph theory and algorithms needs to be developed to enjoy the subject. Computer software or a program without adequate knowledge of mathematics is often difficult to understand. The entire structure of an algorithm can be understood through a flow chart or its graph (or digraph). Graphs (or digraphs) are thus much more useful for a better understanding of structurally dependent algorithms and/or outputs. Digraphs based on congruence equations are a primary interest in the field of discrete mathematics for number theorists and computer scientists. Power digraphs have a broad range of applications that are easily recognizable in almost every field and presented with the basic properties of integers. For instance, if a typical power digraph is described and its loops are discovered. Then, instead of ordinary integers, one can consider these vertices (that is, loops) in forming a new cipher. Obviously, it would be difficult to decode this type of cipher unless one knows the correct mapping and its reverse mapping (if possible). In fact, a typical corresponding congruence must be solved to decode such a produced cipher. The problem of factorization in computer science is a very difficult problem for large integers. For such an integer, a digraph which assigns a component to its divisor can always be described. The divisor number can therefore be enumerated as the number of components that are non-isomorphic. The required integer may therefore be written canonically. Let's define our power digraph before proceeding further.

    Let m>0 be any integer and ¯r denotes the set of all integers which leave remainder r under modulo m, also referred as a residue class of m. Thus, {¯0,¯1,¯2,¯3,,¯m1} is the set of all residue classes of m. This set is called a complete residue system (CRS). We construct a digraph G(p,m) on this set of complete residue classes of m and build a directed edge from u to v if and only if upv(modm). The vertices u1,u2,...,us will form a cycle of length s if

    up1u2(modm),up2u3(modm),upsu1(modm). (1.1)

    The indegree of a vertex a is the count of edges incident with it and the number of edges leaving from a vertex a is called outdegree of a. The indegree and outdegree are labeled as indeg(a) and outdeg(a), respectively. The cycles of length one are called fixed points of the map f(u)=up and cycles of length q are called q-cycles. A maximal connected simple subetaaph of the corresponding digraph G(p,m) is termed as a component. Since every integer lies in a unique residue class of m, so its outdegree must be one.

    If indegrees and outdegrees of all vertices are the same then the corresponding digraph is called a regular digraph. In this case, a digraph is regular if the indegree of every vertex is one (since the outdegree of each vertex is already one). If the indegrees of all vertices are either a positive integer d or 0, we then call it a semiregular digraph. Let G1(p,m) and G2(p,m) denote the subdigraphs induced by the proposed digraph G(p,m) over the set of vertices which are either coprime to m or not, respectively. Note that the digraphs G1(p,m) and G2(p,m) are disjoint and their union is G(p,m). The digraph G(11,17) is depicted in Figure 1.

    Figure 1.  The digraph G(11,17).

    Power digraphs have been of great interest for the last few decades. In 1967, Bryant [1] employed quadratic digraphs and enumerated isomorphic subetaoups of a finite group. Szalay [2] investigated some interesting properties of power digraphs based on congruences and established the existence of cycles in components. In 1996, Rogers [3] and Somer et al. [4] investigated the structures of quadratic maps and explored a few results on fixed points, existence of cycles and few decomposition of components. Mahmood and Ahmad [5] established many results for k-array digraphs and completely described an enumeration of squares of 2k using modular arithmetic for any intger k. Aslam and Mahmood introduced and investigated simple graphs over exponential congruences and characterized all cycles and components completely in [6]. Yangjiang et al. [7] and Somer et al. [8] introduced and investigated the symmetric structures (isomorphic components) of such digraphs. For a fixed k, many useful results on loops, cycles, components and symmetry of power digraphs for the congruence equation xky(modm) have been proposed and proved in [9,10,11,12,13]. Akbari [14] established a relation between edge chromatic number of G(R) with the maximum degree of G(R), where G(R) denotes the zero-divisor graph over of a finite commutative ring R. Wei et al. and Rezaei et al. [15,16,17] discussed graphs based on quadratic and cubic congruences over finite integral rings. Carlip and Mincheva [18] defined an M-ordered symmetric digraph of G based on M-size subsets, each containing M-isomorphic components. Deng and Yuan [19] investigated symmetric digraphs for a fixed power modulo n. Meemark and Wiroonsri [20,21] discussed the structure of G(R,k), where R is the quotient ring of polynomials over finite fields and k is the modulus. Mahmood and Ali [22,23,29] investigated new numbers on euler totient, super euler function and labeling algorithm on several classes of graphs with application. Alolaiyan et al. [24] studied non-conjugate graphs associated with finite groups. Portilla et al. [25] generalize the classical definition of Gromov hyperbolicity to the context of directed graphs. It is worth mentioning that the problem of enumeration of components of power digraphs is still open. In fact, previously all structures have been established for a fixed power k. In this piece of work, we generalize the structures of these digraphs when power is an odd prime p. That is, we incorporate the congruence, xky(modm).

    We organize our paper as follows. In Section 1, we introduce our digraph with some important definitions and also provide some new results on fixed points. In Section 2, we prove some results that enumerate cyclic vertices as well as the existence of a t-cycle in G(p,m). Then, we define two subdigraphs G1(p,m) and G2(p,m). Then after, we elaborate cyclic structures and enumerate components of these subdigraphs for m=2r and m=pr for all positive integers r. Finally, we prove that the digraph G1(p,m) consists of p1 isomorphic trees where as G2(p,m) is a tree with root at 0 with indeg(0)=pkkp. In Section 3, we characterize regularity and semiregularity of G1(p,m). We need the following definitions for use in sequel.

    Definition 1.1. [26] Euler totient function counts the positive integers up to a given integer m that are relatively prime to m. It is written using the Greek letter phi as ϕ(m), also called Euler phi function.

    Definition 1.2. [26] Let m>0 be any integer. For a prime p, the Carmichael λ-function (or λ(m)) is defined as follows: λ(1)=1=ϕ(1),λ(2)=1=ϕ(2),λ(4)=2=ϕ(4),λ(2k)=12ϕ(2k),k3, and λ(pk)=ϕ(pk),k1.

    Theorem 1.3. [27] (Carmichael). Let a,mN. Then aλ(m)1(modm) if and only if gcd(a,m)=1. Here, gcd(a,m) is the greatest common divisor of a and m.

    Theorem 1.4. The Chinese Remainder Theorem (for detail see page 230, Fact 4 of [28])

    Define

    η1={0ifb=0,11ifb2,

    and

    η2={0ifb<31ifb3,

    If gcd(2b,u)=1, then the number of solutions for the congruence uta(mod2b) is either 0 or (gcd(2,t))η1(gcd(2b2,t))η2.

    The following inequality can easily be proved using mathematical induction.

    Lemma 1.5. For t2,tβ(t1),β=2,3,4,.

    Lemma 1.6. For a prime p of the type p3(mod4),k4, then, 1,2k1±1,2k1 are fixed points in G1(p,2k) and 0 is the only fixed point in G2(p,2k).

    Proof. For k4,α=1+2k1 is a fixed point if αpα(mod2k). For this, note that

    (1+2k1)p=1+p2k1+pβ=2(pβ)2β(k1). (1.2)

    As k4, Lemma 1.5 invokes, kβ(k1),β=2,3,4,,n which further gives 2k|2β(k1). But then,

    pβ=2(pβ)2β(k1)0(mod2k). (1.3)

    Also, p=4t+3 for some integer t=0,1,2,. Using the expression of p together with Eqs (1.2) and (1.3), we get

    (1+2k1)p1+(3+22t)2k1(mod2k)1+(1+2+22t)2k1(mod2k)1+(2k1+2k+2k+1)(mod2k)1+2k1(mod2k).

    Similarly, we can follow the same procedure to prove the remaining fixed points. Also by Theorem 1.4, the number of solutions of up11(mod2b)with2u is (gcd(2,p1))η1(gcd(2b2,p1))η2. Note that, in our case, η1=η2=1,withb3. As p3(mod4) can also be written as p1=2(1+2t),tZ+. Hence, we get gcd(2,2(1+2t))gcd(2b2,2(1+2t))=22=4. This shows that there are exactly four fixed points.

    Proposition 1.7. The graph G1(p,2k) has 8 fixed points which are 1,±1+2k1,±1+2k2,2k1,(2k2±1)+2k and G2(p,2k) contains only 0 as a fixed point where k4 and p5(mod8).

    Proof. For k4,α=1+2k2 is fixed point if αpα(mod2k). For this, note that

    (1+2k2)p=1+p2k2+pβ=2(pβ)2β(k2). (1.4)

    As k4, Lemma 1.5 invokes, k4,kβ(k2),β=2,3,4,,n. Then 2k|2β(k2). Hence

    pβ=2(pβ)2β(k2)0(mod2k). (1.5)

    Also, p=8t+5 for some integer t=0,1,2,. Using p together with Eq (1.5) in (1.4), we get

    (1+2k2)p1+(5+8t)2k2(mod2k)1+(1+22+23t)2k2(mod2k)1+(2k+2k2+2k+1t)(mod2k)1+2k2(mod2k).

    The remaining fixed points can be proved in a similarly technique. Also by Theorem 1.4, the number of solutions of up11(mod2b)with2u is (gcd(2,p1))η1(gcd(2b2,p1))η2. In our case, take η1=η2=1,withb4. As p5(mod8) implies that p1=4(1+2t),tZ+. Hence, we get gcd(2,4(1+2t))gcd(2b2,4(1+2t))=24=8. This shows that there are exactly eight fixed points.

    Figure 2 reflects Proposition 1.7.

    Figure 2.  The digraph G(29,25).

    The vertices v1,v2,v3,,vt compose a component if for every j,1jt, there exists some i,1it, such that vpjvpi(modm), for all ji. By [8], it has been established that there exists one and only one cycle in every component of such digraphs. While the enumeration of components is still an open problem. In this section, an enumeration of cycles and components (up to isomorphism) of G(p,2k) is proposed for certain classes of p. Also, we examine all integers for which there are p number of components. The following theorem also validates a similar result given in [4] for a quadratic congruences.

    Theorem 2.1. For an odd prime p, define m=2ipt,i=0,1,2,t1. Then G(p,m) contains an s-cycle if and only if ps1(modd) for a smallest integer s>0 provided d>0 with dλ(m).

    Proof. Let m=2ipt,i=0,1,2,t1 and suppose G(p,m) contains an s-cycle. Assume that u is an arbitrary vertex on this cycle. Then upsu(mod2ipt) for a smallest integer s>0. That is, u(ups11)0(mod2ipt). Clearly, gcd(u,ups11)=1. Thus if we let m1=gcd(u,m) and m2=m/m1, then s>0 must be smallest such that u0(modm1) and vps11(modm2). Using Chinese Reminder Theorem, we get a solution x to satisfying x1(modm1) and xa(modm2). Consequently, the integer s>0 is, in fact, least such that xps11(modm1) and xps11(modm2). Both yields that, xps11(mod2ipt). Let d=ordxm(d=ordxm if d is the least positive integer such that xd1(modm)). Then, x1(modm1) enforces that s>0 is the least integer such that ps1(modd). Also, if d=ordxm, then (x,2ipt)=1, so by Carmichael Theorem, it is evident that d|λ(2ipt).

    Conversely, suppose d>0 with d|λ(m) and let u=gλ(2ipt)|d. Then d=ordum. As d|ps1 but dpl1 for 0l<s. We deduce that the integer s>0 is least so that ups11(mod2ipt). Equivalently, uups1upsu(mod2ipt).

    Theorem 2.2. For any prime p with p3(mod8), the vertices 1+ps2k2,k4 for s=0,1 form a cycle of length 2 in G(p,2k).

    Proof. The vertices α0 and α1 form a cycle of length 2 in G(p,2k) if and only if αp0α1(mod2k),αp1α0(mod2k). Now

    (1+ps2k2)p=1+ps+12k2+pβ=2(pβ)pβs2β(k2). (2.1)

    As k4, Lemma 1.5 invokes, k4,kβ(k2),β=2,3,4,,n. That is, 2k|2β(k2). Therefore,

    pβ=2(pβ)pβs2β(k2)0(mod2k). (2.2)
    (1+ps2k2)p1+ps+12k2(mod2k), (2.3)

    s=0,1 and p3(mod8). Also, p=8t+3 for some integer t=0,1,2,. Using this together with Eq (2.3), for s=1 we get that

    (1+ps2k2)p1+(3+23t)22k2(mod2k)1+(9+26t2+3.24t)2k2(mod2k)1+(1+23+26t2+3.24t)2k2(mod2k)1+(1+23+26t2+3.24t)2k2(mod2k)1+2k2(mod2k). (2.4)

    From Eqs (2.3) and (2.4), we find that the vertices 1+ps2k2 for s=0,1 form a cycle of length 2 in G(p,2k), where k4 and p3(mod8).

    Theorem 2.3. For any prime p such that p3(mod8), the vertices 2k+pps,k>2 for s=0,1,2,3,,2k21 form a cycle of length 2k2 in G(p,2k).

    Proof. The vertices α0,α1,α2,,α2(k2)1 form a cycle of length 2k2 in G(p,2k) if and only if αp0α1(mod2k),αp1α2(mod2k),,αp2(k2)1α0(mod2k). Now

    (2k+pps)p=2k+(pps)p+pβ=2(pβ)(pps)pβ2(β(k)), (2.5)

    Since k>2,kβ(k),β=1,2,3,4,,n. Then 2k|2β(k). Hence,

    pβ=2(pβ)(pps)pβ2β(k)0(mod2k), (2.6)

    putting in Eq (2.5), we get

    (2k+pps)p2k+pps+1(mod2k), (2.7)

    s=0,1,2,3,,2(k2)1 and p3(mod8). Finally, we noted that

    (2k+pp2(k2)1)2k+p(mod2k). (2.8)

    Since, p2(k2)11(mod2k), for any k>2 it implies that,

    pp2(k2)1p(mod2k).

    Eqs (2.7) and (2.8) give that vertices 2k+pps,k>2 for s=0,1,2,3,,2(k2)1 form a cycle of length 2k2 in the graph G(p,2k), where p3(mod8).

    Theorem 2.4. For any prime p such that p3(mod8), the vertices 2k+(p+2)ps,k>2 for s=0,1,2,3,,2k21 form a cycle of length 2k2 in the graph G(p,2k).

    The proof is on similar lines as illustrated in the proof of Theorem 2.3.

    The following result is a simple consequence of last two theorems.

    Corollary 2.5. For any prime p such that p3(mod8), the graph G(p,2k) has cycle of length 2k(r+2), where k>2,0rk3.

    In the following theorem, we find all integers for which there are p components.

    Theorem 2.6. (1) The number of components of G(p,m) is p if m=pk for some positive integer k and p is an odd prime.

    (2)The number of components of G(p,m) is p if m is prime of the form m=(p1)×pk+1 for some positive integer k, where p3(mod4).

    Proof. (1) If m=pk then by [[8], Theorem 6.6 on page 2005], we have exactly p fixed points. Now these are either isolated or the roots of their respective components. Thus, if m is any number for which we have more than p components then there must be a cycle of length s>1. But then by Theorem 2.1, s is the least positive integer such that ps1(modd), where dλ(pk) and d>0. That is, dps1 and dλ(m)=(p1)×pk as well. This clearly enforces that d=p1. But then pk1(modp1) for each value of k. In particular, if 1r<s, then pr1(modp1) as well. This has certainly provided a contradiction against the minimality of s. Thus, this case is not all possible. Consequently, G(p,m) has p components.

    (2) If m is prime of the form m=(p1)×pk+1 for some positive integer k, where p3(mod4). Then it can easily be seen that there are p fixed points by [[8], see Theorem 6.6 on page 2005] and by similar argument as part 1 there does not exist cycle of length greater than 1. Thus, there are exactly p components.

    If d is the least positive integer such that nd1(modm) then d will be termed as order of n modulo m, it is denoted as d=ordnm. In the following result, we show that the digraph G1(p,2k),k>0 contains only the cycles of lengths which are the powers of 2 (excluding fixed points) and G2(p,2k) form a tree with root 0.

    Theorem 2.7. For any positive integer k, the digraph G1(p,2k), contains cycles of lengths as integral powers of 2. That is, the length of any cycle in G1(p,2k) must be of the form 2t,tZ+ and t<k (excluding fixed points) while G2(p,2k) form a tree with root 0. Moreover, indeg(0)=2kkp.

    Proof. It is well known that there must be an equal number of residues of m=2k which are prime to m and those which are not prime to m. Thus the digraphs G1(p,m) and G2(p,m) contains equal number of vertices 2k1, by Lemma 1.6. It can be seen that, ±1+2k1,1+2k,1, are the only fixed points of G1(p,2kp), where p3(mod4), and by Proposition 1.7. G1(p,2k) has 8 fixed points which are 1,2k1±1,±1+2k2,2k1,(2k2±1)+2k, where p5(mod8), and if p=2ua+1 (p1mod8) with an odd a, then G1(p,2k) contains 2u+1 fixed points, the elements of order dividing 2u, provided that ku+2. By Theorem 2.1, there would be a cycle of length s if and only if s=ordpd, for some divisor d of λ(m)=2k2. Now if there exists such a cycle, then s being order of p modulo a divisor of 2k1 must be of the form 2t, for some integer t>0. As far the other case is concerned, we note that all, even residues of 2k, will be connected by a tree. Thus, (2kkp) numbers, namely 2kp,22kp,32kp,,2kkp2kp are mapped onto 0. Consequently, indeg(0)=(2kkp).

    In the following theorem, we investigate the structure of isomorphic trees.

    Theorem 2.8. Let t be any positive integer and m=pt. Then the digraph G1(p,m) consists of p1 isomorphic trees. Moreover, G2(p,m) is a tree with root at 0 and indeg(0)=pttp.

    Proof. We know that the digraph G(p,m) has exactly p components with p fixed points (For detail, see Theorem 6.6 on page 205 in [8]). Note that pttp elements, namely, ptp,2pttp,3pttp,,pttpptp are adjacent to 0 in G2(p,m). Also, p|ϕ(m)=(p1)pt1, by Theorem 3.3, we obtain that the digraph G1(p,m) is semiregular and every vertex, either has degree 0orp. It is clear that this digraph has a tree with root 0. Now assume the set of non-zero fixed points as {1,a2,a3,,ap12,m1,ma2,ma3,,map12}. Define T1,Tm1,Ta2,Tma2 so on Tap12 and Tmap12 trees containing the fixed points {1,m1,a2,ma2,a3,ma3,,ap12,map12}, respectively. we can easily deduce that T1Tm1, Ta2Tma2,,Tap12Tmap12. Now, if we multiply each vertex of the tree T1 by number a2, we have tree Ta2. Similarly, if we multiply each vertex of the tree T1 by number a3, we have a tree Ta3. By continuing this fashion if we multiply T1 by ap12 have a tree Tap12. This is possible if gcd(ai,m)=1, where i=1,2,3,,ap12. Consequently, it yields that, T1Ta2,T1Ta3,,T1Tap12.

    Figures 3 and 4 reflect Theorem 2.7 and Theorem 2.8, respectively.

    Figure 3.  The digraph G(11,26).
    Figure 4.  The digraph G(11,112).

    Now, we discuss the components of the digraph G(p,m). The notation At(G(p,m)) denotes the number of cycles of length t in the digraph G(p,m).

    Theorem 2.9. For any prime p such that p3(mod8), the graph G(p,2k) has 11+4(k5) components, where k>4.

    Proof. To prove this result, first we find the number of cycles of length 2k(r+4), where k>4,0rk5 and p3(mod8). By Lemma 1.5, it is found that A1(G(p,2k))=5 and obtained the number of cycles of length two A2(G(p,2k)) by using Theorem 6.6 of [8] for δi=2, given as

    A2(G(p,2k))=12[(2gcd(λ(2k),p21)+1)d|2d2dAd(2k,p)].

    Now gcd(λ(2k),p21)=8, where p3(mod8),k>4 and A1(p,2k)=5, hence, A2(G(2k,p))=6

    A22(G(p,2k))=122[(2gcd(λ(2k),p221)+1)d|22d22dAd(p,2k)],

    gcd(λ(2k),p221)=24=16,p3(mod8),k>4, A1(p,2k)=5 and A2(p,2k)=6.

    A22(G(p,2k))=4.A23(G(p,2k)=123[(2gcd(λ(2k),p231)+1)d|23d23dAd(p,2k)].

    gcd(λ(2k),p231)=25=32,p3(mod8),k>4, A1(p,2k)=5, A2(p,2k)=6, and A3(p,2k)=4.

    A23(G(p,2k))=4.
    A2k4(G(p,2k))=12k4[(2gcd(λ(2k),p2k41)+1)d|2k4d2k4dAd(p,2k4)],

    gcd(λ(2k),p2k41)=2k2,p3(mod8),k>4, A1(p,2k)=5, A2(p,2k)=6,A22(p,2k)=A23(p,2k)==A2k4(p,2k)=4.

    A2k4(G(p,2k))=4.

    So by counting principle adding A1(p,2k)=5, A2(p,2k)=6,A22(p,2k)=A23(p,2k),,A2k4(2k,p)=4. We get 11+4(k5) components, for k>4, and p3(mod8).

    Theorem 2.10. For any prime p such that p5(mod8), the graph G(p,2k) has 13+4(k5) components, where k>4.

    The proof is on similar lines as illustrated in the proof of Theorem 2.9.

    Theorem 2.11. Let p be any prime such that p7(mod24). Then graph G(p,2k) has 19+8(k6) components, where k>5.

    The proof is on similar lines as illustrated in the proof of Theorem 2.9.

    Figure 5(a) and (b) reflect Theorem 2.9.

    Figure 5.  The digraph G(19,27).

    In this section, we give conditions for the regularity and semiregularity of our proposed graph. In the following result, we characterize the regularity of the digraph G1(p,m).

    Lemma 3.1. The digraph G1(p,m) is regular if and only if pϕ(m), where ϕ is the Euler's function.

    Proof. We suppose that G1(p,m) is regular. The regularity of G1(p,m) yields that the indeg(v)=1 for every vertex v in G1(p,m). This means that xpv(modm) has a unique solution. Without loss of generality, assume v1(modm) and let β be the unique solution of the congruence xp1(modm). That is, βp1(modm). Now, if pϕ(m) then ϕ(m)=pt for some integer t. Note that, t=1 is impossible as ϕ(m) is always even. Also by Euler Theorem, βϕ(m)1(modm) as (β,m)=1 (by definition of G1(m)). Then, βpt1(modm) or (βt)p1(modm). This shows that βt,t>1 is another solution of xp1(modm). This means that indeg(1)=2, a contradiction against the fact that G1(p,m) was regular. Therefore, pϕ(m). Conversely, let pϕ(m) and we suppose that G1(p,m) is not regular. Then there must be at least one vertex α such that indeg(α)>1. For the sake of convenience, take α=1 with indeg(α)=2. This means that xp1(modm) has two solutions. Let these be α and αt,t>1. Then, αp1(modm) and αpt1(modm). But, αϕ(m)1(modm). Hence, we deduce that either ϕ(m)=p or ϕ(m)=pt. As ϕ(m) is always even, so ϕ(m)=pt. That is, pϕ(m), a contradiction.

    Lemma 3.2. Let m>0 be any square free integer and p be any odd prime. The digraph G(p,m) is cyclic if and only if pϕ(m).

    Proof. Recall that a digraph is cyclic if all of its components are cycles. Also, every regular digraph is cyclic. Hence, by Lemma 3.1, G1(p,m) is cyclic if and only if pϕ(m). For G2(p,m), suppose pϕ(m) and let α be any vertex in G2(p,m). Let p be an odd prime such that pgcd(α,m). Then we can find integers r and s such that α=rp and m=ps with gcd(r,s)=1. Now if β is the solution of the congruence xpα(modm), then βpα(modm) yields that βp=α+mt for some integer t. But then βp=rp+spt. Consequently, pβ such that pgcd(α,m). This means that βpα0(modp). Thus we conclude that a number β exists such that it is a solution of xpα(modm). Next we show that this solution is unique modulo m. Since pϕ(m), so gcd(p,ϕ(p))=1. Then the linear congruence py1(modp1) has a unique solution in y. Finally, we put βαy(modp) to get βpαp.ya(modp). By Chinese Reminder Theorem, we get that β is a unique solution of xpα(modm). Thus, indegree of this arbitrary vertex is one. This certainly implies that every vertex is either a loop (a cycle of length one) or at some cycle. The converse is a direct consequence of Lemma 3.1.

    For further result on semiregularity, we define a function η as,

    η(m)={ηo(m)+1ifp2|mηo(m)ifp2m,

    where ηo(m) is the number distinct prime divisors of m such that p1(modp). In the following theorem, we characterize the semiregularity of G1(p,m), where p is an odd prime.

    Theorem 3.3. The digraph G1(p,m) is semiregular if and only if p|ϕ(m), Also the indegrees in G1(p,m) are either pη(p) or zero.

    Proof. By definition of digraph G1(p,m), it is indicated, that βϕ(m)1(modm) for each vertex β in G1(p,m). This means that the indegrees of the vertices of G1(p,m) are same if indeg(β)>0. To find the indeg(β)>0, we just count the indegrees of 1. For this purpose we just count the number of solutions of the congruence, xp1(modpr). Let p be an odd prime and r be any positive integer. Then we see that, (pr1+1)p1(modpr). Likewise, we see that the numbers, 2×pr1+1,3×pr1+1,4×pr1+1,5×pr1+1,6×pr1+1,,p×pr1+1 also solutions of the congruence, xp1(modpr). While modulo p, there are always p solutions whenever p1(modp) and there is a trivial solution if p1(modp) (for detail see [26], page 104). Using the canonical representation of m into odd primes and Chinese Remainder Theorem, simultaneously, we must get that αp1(modm) either have pη(m) solutions or have no solution for each vertex α in G1(p,m). On the other hand, we let G1(p,m) is semiregular and indeg(α)=pη(m) for αG1(p,m). This means that αp1(modm). Using multiplicative order and Euler Theorem for α, we deduce that pϕ(m). Conversely, assume that pϕ(m). then by Lemma 3.2 indegree of each vertex is one which is contradiction. Hence pϕ(m).

    Figure 6 reflects Theorem 3.3.

    Figure 6.  The digraph G(11,67).

    Until the date, several papers on power diagraphs have been published for the fixed power of the congruences modulo an integer. For example, power diagraphs corresponding to x2=y(modm), x3=y(modm) or else fixed powers have been discussed earlier. In this work, we discussed and generalized the results of power diagraphs for any odd prime p as x power rather fixing. That is, for the congruence of xp=y(modm), where p is any odd prime. We addressed the number of loops, cyclic structures, tree structures and the enumeration of components over residue classes of integers. In Section 1, the fixed points of such diagraphs are described and enumerated. These fixed points are referred to as loops. The existence and enumeration of cycles along with their sizes are discussed in Theorems 2.1–2.4 and in Corollary 2.5. In Theorems 2.6–2.11, we have discussed the enumeration of components and trees for classes of integers. These findings have also been shown in Figures 25 for better comprehension and confirmation. Finally, the results on regularity and semi-regularity are discussed and generalized in Section 3. In fact, we have fully established and defined the desired ideas for power diagraphs with an odd prime power. We believe that the characterizations can be built on the basis of these findings for all composite modules, which can serve as a basis for solving many difficult and open challenges.

    This work was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah. The authors, therefore, acknowledge with thanks DSR for technical and financial support.

    The authors declare that they have no conflict of interest regarding the publication of the research article.



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