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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
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 * A345454 A271684 A194682
KEYWORD
nonn
AUTHOR
R. H. Hardin, Aug 10 2008
STATUS
approved

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Last modified February 29 19:20 EST 2024. Contains 370428 sequences. (Running on oeis4.)