Add skew Hadamard matrix of order 236

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

@@ -2402,6 +2402,11 @@ REFERENCES:
             code, by I.A.Faradjev (with contributions by A.E.Brouwer,
             D.V.Pasechnik). https://github.com/dimpase/coco
 
+.. [FKS2004] \R. J. Fletcher, C. Koukouvinos and J. Seberry.
+            *New skew-Hadamard matrices of order 4·59 and new D-optimal designs of order 2·59*,
+            Discrete Mathematics 286(3) (2004): 251-253.
+            :doi:`10.1016/j.disc.2004.05.009`
+
 .. [FL2001] David Forge and Michel Las Vergnas.
             *Orlik-Solomon type algebras*. European J. Combin.
             **22** (5), (2001). pp. 699-704.

src/sage/combinat/matrices/hadamard_matrix.py

@@ -2014,7 +2014,8 @@ def GS_skew_hadamard_smallcases(n, existence=False, check=True):
     Namely, it needs 4 circulant matrices with extra properties, as described in
     :func:`sage.combinat.matrices.hadamard_matrix.williamson_goethals_seidel_skew_hadamard_matrix`
     Matrices for `n=36` and `52` are given in [GS70s]_. Matrices for `n=92` are given
-    in [Wall71]_. Matrices for `n=188` are given in [Djo2008a]_.
+    in [Wall71]_. Matrices for `n=188` are given in [Djo2008a]_, and matrices for `n=236` are 
+    found in [FKS2004]_.
     
     Additional data is obtained from skew supplementary difference sets contained in 
     :func:`sage.combinat.designs.difference_family.skew_supplementary_difference_set`, using the 
@@ -2052,6 +2053,10 @@ def pmtoZ(s):
               '++--+---+------++------++-+-++--+-+-+----+---++',
               '+-+-++---++-+---+++---++-++-++-++-+++++-+-+----',
               '+++-++-+-+---+-+++++--+-----++---+--+++++--++-+'],
+        236: ['+-+---+-+-++-++---+----++-----+++++--++++-+++--+--+-+-+++-+',
+              '+-+---+-+-++-++---+----++-----+++++--++++-+++--+--+-+-+++-+',
+              '+++-++----+++-+-+++--+--++------+---+-----+--+-+--+---+----',
+              '++++++--+++--+---++-+-+-+---+-+----++++-++-+--++-+--+------']
     }
 
     if existence: