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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A227258 Number of n X n 0,1 arrays indicating 2X2 subblocks of some larger (n+1)X(n+1) binary array having a sum of two or less, with rows and columns of the latter in lexicographically nondecreasing order.

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%I A227258 #7 Jul 23 2025 05:35:26
%S A227258 2,9,98,2201,112349,13855163,4307345460
%N A227258 Number of n X n 0,1 arrays indicating 2X2 subblocks of some larger (n+1)X(n+1) binary array having a sum of two or less, with rows and columns of the latter in lexicographically nondecreasing order.
%C A227258 Diagonal of A227263
%e A227258 Some solutions for n=4
%e A227258 ..1..1..1..0....1..1..1..1....1..1..1..1....1..1..1..0....1..1..0..0
%e A227258 ..1..0..0..1....1..0..1..1....1..0..1..1....1..0..0..0....1..1..1..0
%e A227258 ..0..1..1..1....1..1..0..1....1..1..1..0....1..0..0..1....1..1..1..1
%e A227258 ..0..0..1..1....1..1..0..0....0..1..0..0....0..0..0..1....0..1..1..1
%K A227258 nonn
%O A227258 1,1
%A A227258 _R. H. Hardin_ Jul 04 2013

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