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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.

A199640 Number of n X n 0..2 arrays with values 0..2 introduced in row major order and each element equal to one or two horizontal and vertical neighbors.

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%I A199640 #8 Jul 22 2025 13:20:46
%S A199640 0,3,112,29229,32177372,176701683386,4539946639286136
%N A199640 Number of n X n 0..2 arrays with values 0..2 introduced in row major order and each element equal to one or two horizontal and vertical neighbors.
%C A199640 Diagonal of A199647
%e A199640 Some solutions for n=3
%e A199640 ..0..1..1....0..0..1....0..1..1....0..0..0....0..0..1....0..0..1....0..1..1
%e A199640 ..0..1..0....1..0..1....0..0..1....0..1..2....0..1..1....0..2..1....0..2..1
%e A199640 ..2..2..0....1..2..2....2..2..1....1..1..2....0..0..0....0..2..1....0..2..2
%K A199640 nonn
%O A199640 1,2
%A A199640 _R. H. Hardin_ Nov 08 2011

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