# The Software for Constructing Trails with Local Restrictions in Graphs

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1 Open Journal of Discrete Mathematics, 203, 3, Published Online April 203 ( The Software for Constructing Trails with Local Restrictions in raphs Tatyana Panyuova, Igor Alferov Department of Mathematical Methods in Economics and Statistics, South Ural State University, Chelyabins, Russian Federation Received December 8, 202; revised March 5, 203; accepted April 20, 203 Copyright 203 Tatyana Panyuova, Igor Alferov This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original wor is properly cited ABSTRACT The present research considers the problem of covering a graph with minimal number of trails satisfying the pre-defined local restrictions The research is devoted to the problem of graph covering by minimal number of trails satisfying some local restrictions Algotithm of allowed Eulerian cycle construction is considered The authors showed that it is possible to recognize the system of transitions and solve the problem of constructing the allowable path by linear time It s also possible to find allowable Eulerian cycle for Eulerian graph or to proclaim that such a cycle does not exist by the time O V E All presented algorithms have the software realization Keywords: Eulerian raph; Trail; Transition; Compatible Path; Algorithm Introduction Lots of problems of finding paths satisfying the different restrictions can be applied to some practical problems For example for sheet material cutting problem plane graph represents the model of cutting plan, and a path covering all its edges defines the trajectory of cutter The restriction defined for this problem is lac of intersection of any initial part of path with edges that are not passed yet [] Creating the control systems using non-oriented graphs the following problems of constructing the paths with different restrictions can arise Among them are straight-ahead paths [2]; paths the next edge of which is defined by the given cyclic order on the set of incident edges [3-5]; paths for which it s necessary to pass some edges in pre-defined order [5] The restrictions on the order of vertices and edges can be classified as local (the next edge of a path is defined by conditions established at the current vertex or edge [2-8]), and global (Eulerian, Hamiltonian cycles, bidirectional double tracing etc) Most of researches are devoted to algorithms with local restrictions of edges order in a path The present research considers the problem of covering a graph with minimal number of trails satisfying the pre-defined local restrictions 2 Constructing of T -Compatible Path The generalization of most of particular cases for problem of simple trail with local restrictions construction and analysis of its computing complexity is made by SSzeider [7] Let s quote the basic definitions and results of this research to mae the further statements clear Let s confine with finite simple graphs Let s designate as V and E the sets of vertices and edges of graph correspondingly For vertex vv let s define the set E V of all graph edges incident to vertex v The degree of vertex v let be designated as ; for : vv dv dv d 0 let Vd d Let H if H be vertex-induced subgraph of graph ie the subgraph received of graph by deleting of one set of vertices and only all edges incident to vertices of this set Restrictions for paths in graph can be defined in terms of allowed transitions graph Definition Let transition graph T v for vertex v V be a graph vertices of which are the edges incident to vertex v ie VT v Ev, and set of edges consists of allowed transitions Definition 2 The system of allowed transitions (or shortly, system of transitions) T is called the set T v v V where T v be the transition graph for vertex v Definition 3 The path P v0ev ev for graph is called T if ei, ei ET v i for each i i Copyright 203 SciRes

2 T PANYUKOVA, I ALFEROV 87 Theorem [S Szeider] If all graphs of transitions belong either class M of full multipartite graphs or class P of matchings then the problem of T trail constructing can be solved by the time O E Otherwise this problem is NP-complete If the system of transitions for a vertex v V is a matching then this problem can be reduced to the problem for graph : V V v, E E E v vv i j: vv i, vvjet v If for any vertex vv graph T v is full multipartite graph then a trail can be constructed by the following algorithm Algorithm T -Compatible Path Input: raph V, E; Vertices x, y the end-vertices of T trail; System of transitions T : vv T v M Output: The sequence of edges corresponding to T trail between vertices x and y or the message that such a path does not exist Step If vertex x or vertex y is isolated then stop: path does not exist Step 2 Delete all isolated vertices from graph Step 3 Construct the supplementary graph as following (Figure ): Each vertex v V v, v2,, vpv where of graph T should be split into vertices p v be the number of parts v The edges of corresponding part of graph T v and one additional vertex v pv are incident to vertex v p ; Add two new vertices w v and w2 v, edge wvw2 v, and edge v pv wj v for each part of graph T v, j 2 Step 4 Construct the initial matching for graph M v v w v p p w2v vv p,2,, pv Step 5 Find the alternate sequence between vertices x and y that enlarges the cardinality of matching for graph If it s impossible to find such a sequence then stop (matching M has maximal cardinality and graph has no T path) Otherwise all the edges of found enlarging path except of additional edges of graph produce the T path between vertices x and y Stop Let s admit that there is open question in research [7] This question is about recognition the multipartiteness of graphs T v Problems of constructing the allowed path or set of paths covering all the edges of given graph are not also considered Let s illustrate an example of graph (Figure 2) that algorithm T -COMPATIBLE PATH cannot be used for constructing of paths covering all edges of graph Let the following system of transitions T is defined for the graph:,, vv, vv, vv 2 vv 5, vv 6 vv 4, vv 8 3, vv 3 2, vv 3 2, vv 2 8, 5 2, 2 vv 4, vv 4 6, vv 7 4, vv 4 3, 2 5, 5 8 vv 2 8, vv 8 5, vv 3 8, vv 8 7, 3 7, 7 8 vv 4 7, vv 7 6, vv 4 6, vv 6 7, 6, 6 5 vv 5, vv 5 6 Supplementary graph for finding of T path between vertices v and v7 which construction is reviewed on step 3 of algorithm is shown at Figure 3 The initial matching M is mared by thic lines The alternate enlarging sequence of edges for this matching be v,v 5,2, v5,2v 5,2, v5,2w5,2 v 5,v5,, v5,v 6,2, 6,2 6,2 w6,v 6,, v6,v6, w5,2w5, 5, 5, v 6,2w6,2 w6,2w6, v6,v7,2,, w v, v v,,,, Edges of this sequence not belonging the initial matching are represented by dash line These edges form the set Figure Illustration how supplementary graph structed is con- Figure 2 Example of graph Copyright 203 SciRes

3 88 T PANYUKOVA, I ALFEROV Figure 3 raph ' received of graph by additional constructions v,v 5,2, v 5,2w5,2, w5,v 5,, v5,v 6,2, v6,2w6,2 v6,w 6,, v6,v7,2, All edges of this set belonging to graph ie v, v 5, v5, v6, v6, v7 form T path from vertex v to vertex v 7 Using algorithm T -COMPATIBLE PATH it s possible to construct only a simple trail between two different vertices (ie a trail where each vertex is presented only once) Figure 4 shows the software realization of the represented algorithm The bold line mars the found trail between vertices and 4 This trail corresponds the system Figure 4 Software for compatible path algorithm of allowed transitions (see the additional window at right bottom side) However in common the direct use of this algorithm does not allow to solve the problem of T path with maximal number of edges constructing Actually the matching of maximal cardinality for graph cannot contain the pairs of edges forming forbidden transition because these edges are incident to one comm on vertex of graph At the same time, in common there may exist T path containing such a pair of edges For example, for graph presented on Figure 2 the path vv, vv, vv, vv, vv, vv in principle cannot be received by constructing the matching of maximal weight for graph This path begins from edge vv 2 and ends by edge vv 5 2 These edges form forbidden transition vv 5 2, v2v, consequently, graph does not contain the alternate path with both of these edges Thus, the question of multipartite T v graph recognition is still open as well as the pro blem of allowed path constructing or finding the set of paths covering all the edges of initial graph 3 Algorithm for Constructing of Compatible Eulerian Trails In the previous section the restrictions for paths were Copyright 203 SciRes

4 T PANYUKOVA, I ALFEROV 89 stated in the terms of allowed transitions system [7] It s shown that the problem of constructing the allowed path in graph can be solved by polynomial time if a system of transitions T consists only of matchings and full multipartite graphs It s trivial to recognize if graph of allowed transitions belongs to the class of matchings If we want recognize if transitions graph belongs to class of full multipartite graphs it s expedient to use the definition of partition system [3-5,8] The conception of partition system is used for definition of allowed trail in terms of forbidden transitions Definition 4 Let V, E be a graph Let P v be some partition of set E v Then the partition system of graph be the system of sets P : P v v V Definition 5 Let p P v, e, f p A trail not containing the transitions ev f and f v e can be called P -comp atible, and transitions ev f and f ve are forbidden Let's admit that graph of allowed transitions T v unambiguously defines the graph of forbidden transitions T v which is the complement of allowed trans itions graph to full graph Thus, using definitions -3 the problem either with use of allowed transitions or forbidden transitions can be stated So the partition system is defined on the set of Ev (the set of vertices incident to v) If edges e and e 2 belong to one subset then edge e 2 cannot be placed after the edge e in a trail Let graph VE, be defined b y the adjacency list Its elements are the structures Each element of this structure consists of two fields: vertex number v i (this vertex is adjacent to the current one); the number of partition element c i To define the degree of the current vertex it is enough to count the number of elements of adjacency list Let s admit that each edge e belongs to two adjacency lists of vertices v i and v j (the ends of an edge) But for each vertex edge e belongs to different partition systems Input data are represented by the following list vector < list < pair <string, int> > > raph; All data are represented by a vector of vertices, each el ement of this vector is a list of pairs The first pair of each list is a vertex number, the second one is its degree The other pairs represent the numbers of adjacent vertices and number of corresponding partition set On the other side, graph of allowed transitions defined by partition system P cannot be arbitrary, and belongs to class M of full multipartite graphs: the elements of partition P v define the parts of graph T v M, and set of its edges E T v e, f E v : p P v e, f p s raph of forbidden transitions T v in this case will consist of P v cliques, this fact can be used for recognition if T v M using algorithm [9] As it was considered earlier, algorithm of S Szeider in common does not allow constructing of allowed trails having maximal length The most interesting are allowed Eulerian trails The necessary and sufficient condition for P trails existence is proved by the following theorem [8] Theorem 2 [A Kotzig] Connected Eulerian graph has P Eulerian trail if and only if vvppv p dv 2 Obviously, complexity of checing the condition of existence of P Eulerian trail is not more than O E Let s list the algorithm for construction of compatible trail Algorithm P-Compatible Eulerian Trail Input data: Eulerian graph V, E, Transitions system P v v V Output data: Allowed Eulerian cycle Step Let 0, Step 2 Find a vertex v for wh ich d v 2 Step 3 Find element of partit ion system containing maximal number of edges I t s enough to loo through the adjacency list of current vertex v and count how many times each element of partition meets at this list Choosing this element we get a class C P v : C max C C P v Step 4 Find any edges e v C and e2v E v C If it s possible choose edges e and e2 incident to vertices of degree more than 2 If set E v C then stop: there is no P Eulerian trail Otherwise go to step 5 Step 5 Construct graph by detaching vertex v, to which only edges e and e 2 are incident, from veredges are ept incident to vertex v tex v The other Step 6 Let class C2 P v contains the edge e 2 v Exclude vertices v and v 2 from partition system Define P v : P, 2 v C C For further modification of partition system define the following Step 6 All partition systems not containing vertex v are taen to the modified system without any changes Step 62 If systems C and C 2 had been consisted of one edge C C2 th en P v: P v Step 63 If C C then 2 : P v P v C e v Step 64 If C2 then Copyright 203 SciRes

5 90 T PANYUKOVA, I ALFEROV :, 2 P v P v C e v C e v 2 Step 65 Co nstruct P Step 7 Define the value P xv,2 x 2 E V Let s admit that the number of edges is a constant value and the number of vertices is increased by Step 8 If 0, let and go to step 2 for graph Otherwise go to step 9 Step 9 Choose any vertex v and mar all achievable vertices If there are unmared vertices go to step 0 otherwise stop the received graph is Eulerian trail without forbidden transitions Step 0 et vertices v and v 2 from a list of mared and unmared vertices of graph These vertices are split from vertex v of graph 0 Unite them to one vertex v,2 There we get a modified graph ˆ Let Step Choose edges ev,2 C and e2v,2e v,2 C so thate, e2 Eva nd e, e2 Ev 2 If set E v,2 C then stop: there is no P Eulerian trail Otherwise conby splitting vertex v,2 struct graph from vertex v,2 Only edges e and e 2 are incident to vertex v,2 All other ed ges are incident to vertex v,2 and go to step 9 The following theorem i s proved in [9] Theorem 3 Algorithm P Eulerian trail correctly solves the problem of constructing the P - compatible Eulerian trail That paper also shows that computing complexity of this algorithm is O d v O E V 0,,, Thus, the constructed algorithms are resolved by the polynomial time and can be simply realized using standard computing facilities Let s consider the allowed trail construction for a graph on Figure 5 Let its transitions system is as shown in a table below The numbers in white circles show the number of partition system the edge belongs for each vertex Let s construct allowed Eulerian cycle beginning and ending at vertex At the first iteration the first vertex is split To simplify the illustration let s consider the short version of vertex splitting (the example of full version is presented on Figure 3) Figure 6 and table show graph with split vertex and a list of connectivity where (2,2) (6,) (8,2) (5,) 2 (,2) (6,) (7,) (3,2) 3 (2,) (7,) (5,2) (4,2) 4 (3,2) (5,) 5 (4,) (3,) (8,2) (,2) 6 (,) (2,) (7,2) (8,2) 7 (2,2) (3,) (6,) (8,2) 8 (,2) (6,) (7,2) (5,2) Figure 5 Example (8,2) (5,) 2 (',2) (6,) (7,) (3,2) 3 (2,) (7,) (5,2) (4,2) 4 (3,2) (5,) 5 (4,) (3,) (8,2) (,2) 6 (',) (2,) (7,2) (8,2) 7 (2,2) (3,) (6,) (8,2) 8 (,2) (6,) (7,2) (5,2) ' (2,2) (6,) Figure 6 Th e first itera tion of algorithm new or modified elements are colored by gray Figures 7-2 and tables under them represent all other iterations of algorithm Copyright 203 SciRes

6 T PANYUKOVA, I ALFEROV 9 (8,2) (5,) 2 (7,) (3,2) 3 (2,) (7,) (5,2) (4,2) 4 (3,2) (5,) 5 (4,) (3,) (8,2) (,2) 6 (',) (2',) (7,2) (8,2) 7 (2,2) (3,) (6,) (8,2) 8 (,2) (6,) (7,2) (5,2) ' (2',2) (6,) 2' (',2) (6,) Figure 7 The second iteration of algorithm (8,2) (5,) 2 (7,) (3',2) 3 (7,) (4,2) 4 (3,2) (5',) 5 (3',) (,2) 6 (',) (2',) (7,2) (8,2) 7 (2,2) (3,) (6,) (8,2) 8 (,2) (6,) (7,2) (5',2) ' (2',2) (6,) 2' (',2) (6,) 3' (2,) (5,2) 5' (4,) (8,2) Figure 9 The forth iteration of algorithm (8,2) (5,) 2 (7,) (3',2) 3 (7,) (4,2) 4 (3,2) (5,) 5 (4,) (3',) (8,2) (,2) 6 (',) (2',) (7,2) (8,2) 7 (2,2) (3,) (6,) (8,2) 8 (,2) (6,) (7,2) (5,2) ' (2',2) (6,) 2' (',2) (6,) 3' (2,) (5,2) (8,2) (5,) 2 (7,) (3',2) 3 (7,) (4,2) 4 (3,2) (5',) 5 (3',) (,2) 6 (2',) (8,2) 7 (2,2) (3,) (6',) (8,2) 8 (,2) (6,) (7,2) (5',2) ' (2',2) (6',) 2' (',2) (6,) 3' (2,) (5,2) 5' (4,) (8,2) 6' (',) (7,2) Figure 8 The third iteration of algorithm Figure 0 The fifth iteration of algorithm Copyright 203 SciRes

7 92 T PANYUKOVA, I ALFEROV (8,2) (5,) 2 (7',) (3',2) 3 (7',) (4,2) 4 (3,2) (5',) 5 (3',) (,2) 6 (2',) (8,2) 7 (6',) (8,2) 8 (,2) (6,) (7,2) (5',2) ' (2',2) (6',) 2' (',2) (6,) 3' (2,) (5,2) 5' (4,) (8,2) 6' (',) (7,2) 7' (2,2) (3,) Figure The sixth iteration of algorithm (8',2) (5,) 2 (7',) (3',2) 3 (7',) (4,2) 4 (3,2) (5',) 5 (3',) (,2) 6 (2',) (8',2) 7 (6',) (8,2) 8 (7,2) (5',2) ' (2',2) (6',) 2' (',2) (6,) 3' (2,) (5,2) 5' (4,) (8,2) 6' (',) (7,2) 7' (2,2) (3,) 8' (,2) (6,) In result graph is split into a simple cycle It s possible to construct allowed Eulerian cycle beginning at any its vertex Fo r example, the following cycle beginning at vertex can be constructed: It s possible to recognize the system of transitions and to solve the problem of constructing the allowable path by linear time It s also possible to find P -allowable Eulerian cycle for Eulerian graph or to proclaim that such a cycle does not exist This problem can be solved by the time O V E using algorit hm P - compatible Eulerian trail 4 Acnowledgements The research is supported by the Ministry of Education and Science of Russian Federation, contract 4B REFERENCES [] T Panyuova, Cover with Ordered Enclosing for Flat raphs, Electronic Notes in Discrete Mathematics, Vol 28, 2007, pp 7-24 doi:006/jendm [2] T Pisansi, T W Tucer and A Zitni, Straight-Ahead wals in Eulerian raphs, Discrete Mathematics, Vol 28, No -3, 2004, pp doi:006/jdisc [3] H Fleischner, Eulerian raphs and Related Topics, Part, Vol, Elsevier, Amsterdam, 990 [4] H Fleischner, Eulerian raphs and Related Topics, Part, Vol 2, Elsevier, Amsterdam, 99 [5] H Fleischner, L W Beinee and R J Wilson, Eulerian raphs, Selected Topics in raph Theory 2, Academic Press, London, New Yor, 983, pp 7-53 [6] D Chebiin, On -Edge-Ordered raphs, Discrete Mathematics, Vol 28, No -3, 2004, pp 5-28 doi:006/jdisc [7] S Szeider, Finding Paths in raphs Avoiding Forbidden Transitions, Discrete Applied Mathematics, Vol 26, No 2-3, 2003, pp doi:006/s066-28x(02) [8] A Kotzig, Moves without Forbidden Transitions in a raph, Matematicý Časopis, Vol 8, No, 968, pp [9] T A Panyuova, The Paths with Local Restrictions, Reports of South Ural State University Section: Mathematical Modelling and Programming, Vol 5, No 6, 200, pp (in Russian) Figure 2 The last iteration of algorithm Copyright 203 SciRes

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