sagemathgh-38321: Implemented generators for some small graphs/ digraphs

    
<!-- ^ Please provide a concise and informative title. -->
This PR adds generators for the Bicorn graph, the Tricorn graph, the
Murty graph, the KohTindell digraph, the Cubeplex Graph and the Twinplex
graph.
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
This PR introduces new static methods, specifically `BicornGraph`,
`TricornGraph`, `MurtyGraph`, `KohTindellDiGraph`, `CubeplexGraph` and
`TwinplexGraph`.
<!-- v Why is this change required? What problem does it solve? -->
There is no current implementation of the generators for the above
mentioned graphs/ digraphs.
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->
Fixes sagemath#38320.


### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
Nothing as of now (up to my knowledge).
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->

cc: @dcoudert.
    
URL: sagemath#38321
Reported by: Janmenjaya Panda
Reviewer(s): David Coudert, Janmenjaya Panda

src/doc/en/reference/references/index.rst

@@ -1443,6 +1443,11 @@ REFERENCES:
              for closed Riemannian manifolds*, Ann. of Math. (2) 45 (1944),
              747–752.
 
+.. [CHNP2020] Kieran Clancy, Michael Haythorpe, Alex Newcombe and Ed Pegg Jr,
+              *There Are No Cubic Graphs on 26 Vertices with Crossing Number 10
+              or 11*, Graphs and Combinatorics 36, pages: 1713 -- 1721, (2020),
+              :doi:`10.1007/s00373-020-02204-6`.
+
 .. [CP2023] \M. Cati and D.V. Pasechnik.
             *Implementing Hadamard Matrices in SageMath*.
             Preprint, :arxiv:`2306.16812`, (2023).
@@ -1700,6 +1705,10 @@ REFERENCES:
             bi-matrix
             games*. http://vknight.org/unpeudemath/code/2015/06/25/on_testing_degeneracy_of_games/ (2015)
 
+.. [CKWL2019] Marcelo H. de Carvalho, Nishad Kothari, Xiumei Wang and Yixun
+              Linc. *Birkhoff-von Neumann graphs that are PM-compact*. 2019.
+              :doi:`10.48550/arXiv.1807.07339`.
+
 .. [CL1996] Chartrand, G. and Lesniak, L.: *Graphs and Digraphs*.
             Chapman and Hall/CRC, 1996.
 
@@ -1725,6 +1734,11 @@ REFERENCES:
 .. [CLG1997] Frank Celler and C. R. Leedham-Green,
              *Calculating the Order of an Invertible Matrix*, 1997
 
+.. [CLM2006] Marcelo H. de Carvalho, Cláudio L. Lucchesi and U.S.R. Murty,
+             *How to build a brick*, Discrete Mathematics, Volume 306,
+             Issues 19--20, Pages 2383--2410,ISSN 0012--365X, (2006),
+             :doi:`10.1016/j.disc.2005.12.032`.
+
 .. [CLRS2001] Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest
               and Clifford Stein, *Section 22.4: Topological sort*,
               Introduction to Algorithms (2nd ed.), MIT Press and
@@ -2621,6 +2635,11 @@ REFERENCES:
             Wehler K3 Surfaces over finite fields*. New Zealand Journal
             of Mathematics 45 (2015), 19–31.
 
+.. [FiLi2001] Ilse Fischer and Charles H.C. Little, *A Characterisation of
+              Pfaffian Near Bipartite Graphs*, Journal of Combinatorial Theory,
+              Series B, vol. 82, issue 2, (2001), pages: 175 -- 222, ISSN:
+              0095 -- 8956, :doi:`10.1006/jctb.2000.2025`.
+
 .. [Fil2017] Ivana Filipan, *An Invitation to Combinatorial Tropical Geometry*.
              Conference: 1st Croatian Combinatorial Days. 2017.
              :doi:`10.5592/CO/CCD.2016.05`.
@@ -3943,6 +3962,10 @@ REFERENCES:
             47:95-104, 1997.
             :doi:`10.1023/A:1022444205860`
 
+.. [KM2015] Nishad Kothari and U.S.R. Murty. *K4-free and C6¯-free Planar
+            Matching Covered Graphs.* Journal of Graph Theory. 82. (2015)
+            :doi:`10.1002/jgt.21882`.
+
 .. [KMAUTOM2000] Masayuki Kanda, Shiho Moriai, Kazumaro Aoki, Hiroki Ueda,
                  Youichi Takashima, Kazuo Ohta, and Tsutomu Matsumoto,
                  *E2 - a new 128-bit block cipher*; in IEICE Transactions on
@@ -4484,6 +4507,11 @@ REFERENCES:
              IEEE Trans. Inf. Th. 25(1979), 1-7.
              :doi:`10.1109/TIT.1979.1055985`.
 
+.. [Lov1983] László Lovász,
+             *Ear-decompositions of matching-covered graphs*,
+             Combinatorica 3, 105--117 (1983)
+             :doi:`10.1007/BF02579346`.
+
 .. [LP2007] \G. Leander and A. Poschmann,
             *On the Classification of 4 Bit S-boxes*; in WAIFI, (2007), pp. 159-176.
 
@@ -5164,6 +5192,10 @@ REFERENCES:
             :doi:`10.1007/s00453-006-1225-y`,
             http://www.cs.uoi.gr/~stavros/C-Papers/C-2004-SODA.pdf
 
+.. [NT2007] Serguei Norine and Robin Thomas. *Minimally Non-Pfaffian Graphs*.
+            Combinatorica, vol. 27, no. 5, pages: 587 -- 600, Springer. 2007.
+            :doi:`10.1016/j.jctb.2007.12.005`.
+            
 .. [Nur2004] K. Nurmela. *Upper bounds for covering arrays by tabu search*.
              Discrete Applied Math., 138 (2004), 143-152.
 
@@ -5675,6 +5707,11 @@ REFERENCES:
 .. [RS2012] G. Rudolph and M. Schmidt, "Differential Geometry and Mathematical Physics.
             Part I. Manifolds, Lie Groups and Hamiltonian Systems", Springer, 2012.
 
+.. [RST2019] Neil Robertson, Paul Seymour and Robin Thomas, *Excluded minors in
+             cubic graphs*, Journal of Combinatorial Theory, Series B, vol. 138,
+             (2019), pages: 219 -- 285, ISSN: 0095 -- 8956,
+             :doi:`10.1016/j.jctb.2019.02.002`.
+
 .. [RSW2011] Victor Reiner, Franco Saliola, Volkmar Welker.
              *Spectra of Symmetrized Shuffling Operators*.
              :arxiv:`1102.2460v2`.

src/sage/graphs/digraph_generators.py

@@ -49,6 +49,7 @@
 - Emily A. Kirkman (2006)
 - Michael C. Yurko (2009)
 - David Coudert    (2012)
+- Janmenjaya Panda (2024)
 
 Functions and methods
 ---------------------
@@ -58,6 +59,7 @@
 #                              and Emily A. Kirkman
 #           Copyright (C) 2009 Michael C. Yurko <myurko@gmail.com>
 #           Copyright (C) 2012 David Coudert <david.coudert@inria.fr>
+#           Copyright (C) 2024 Janmenjaya Panda <janmenjaya.panda.22@gmail.com>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -891,6 +893,9 @@ def Circulant(self, n, integers):
         r"""
         Return a circulant digraph on `n` vertices from a set of integers.
 
+        A circulant digraph of order `n` has an arc from vertex `i` to
+        vertex `i+j \pmod{n}`, for each `j` in ``integers``.
+
         INPUT:
 
         - ``n`` -- integer; number of vertices
@@ -899,18 +904,30 @@ def Circulant(self, n, integers):
           that there is an edge from `i` to `j` if and only if `(j-i) \pmod{n}`
           is an integer
 
-        EXAMPLES::
+        EXAMPLES:
 
-            sage: digraphs.Circulant(13,[3,5,7])
-            Circulant graph ([3, 5, 7]): Digraph on 13 vertices
+        Construct and show the circulant graph [3, 5, 7], a digraph on 13
+        vertices::
 
-        TESTS::
+            sage: g = digraphs.Circulant(13, [3, 5, 7])
+            sage: g.show()                          # long time                             # needs sage.plot
+
+        The Koh-Tindell digraph [LM2024]_ is the circulant digraph of order 7
+        with parameters `[1, 5]`.  This `2`-diregular digraph is
+        vertex-transitive but not arc-transitive. The associated bipartite
+        digraph of the Koh-Tindell digraph is a Pfaffian orientation of the
+        Heawood graph. Construct and show the Koh-Tindell digraph::
+
+            sage: kohTindellDigraph = digraphs.Circulant(7, [1, 5])
+            sage: kohTindellDigraph.show()          # long time                             # needs sage.plot
+
+        TESTS:
 
-            sage: digraphs.Circulant(13,[3,5,7,"hey"])
+            sage: digraphs.Circulant(13, [3, 5, 7, "hey"])
             Traceback (most recent call last):
             ...
             ValueError: the list must contain only integers
-            sage: digraphs.Circulant(3,[3,5,7,3.4])
+            sage: digraphs.Circulant(3, [3, 5, 7, 3.4])
             Traceback (most recent call last):
             ...
             ValueError: the list must contain only integers