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A141506 Number of n X n binary matrices, symmetric about the diagonal and under 90-degree rotation, with no more than 2 ones in any 2 X 2 subblock. 1
1, 2, 1, 5, 3, 25, 13, 205, 99, 2849, 1247, 66677, 26615, 2633485, 954340, 175143651, 57733315, 19638725775, 5882854746, 3710411382331, 1010373532123, 1181447019186469, 292421578705864, 633957805831439213, 142628031886778979 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

R. H. Hardin, Table of n, a(n) for n = 0..30

MAPLE

solscount:= proc(vars, cons)

option remember;

   local v1, ve, c, t;

   c, t:= selectremove(type, cons, integer);

   if hastype(c, posint) then return 0 fi;

   if nops(t) = 0 then return 2^nops(vars) fi;

   v1:= vars[1];

   procname(vars[2..-1], subs(v1=0, t)) + procname(vars[2..-1], subs(v1=1, t))

end proc:

A[0]:= 1:

for n from 1 to 25 do

  M:= Matrix(n, n, shape=symmetric);

  m:= ceil(n/2);

  for i from 1 to m do

    for j from i to m do

       M[i, j]:= a[i, j];

       M[j, n+1-i]:= a[i, j];

       M[n+1-i, n+1-j]:= a[i, j];

       M[n+1-j, i]:= a[i, j];

  od od:

  cons:= {seq(seq(M[i, j]+M[i, j+1]+M[i+1, j]+M[i+1, j+1]-2, i=1..n-1), j=1..n-1)};

  vars:= {seq(seq(a[i, j], j=i..m), i=1..m)};

  A[n]:= solscount(vars, cons);

od:

seq(A[n], n=0..25); # Robert Israel, Jun 22 2015

CROSSREFS

Sequence in context: A113178 A108362 A171090 * A271684 A194682 A274105

Adjacent sequences:  A141503 A141504 A141505 * A141507 A141508 A141509

KEYWORD

nonn

AUTHOR

R. H. Hardin, Aug 10 2008

STATUS

approved

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Last modified May 30 12:03 EDT 2020. Contains 334724 sequences. (Running on oeis4.)