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.
%I A188351 #7 Jul 22 2025 11:09:21 %S A188351 9,45,117,565,1953,9153,37269,173409,766869,3589357,16545861,78195505, %T A188351 368573829,1757922765,8394885777,40340645213,194259462205, %U A188351 938930726781,4547344031581,22077243778089,107366485176765,523060579628697 %N A188351 Number of arrangements of n+1 numbers x(i) in -2..2 with the sum of x(i)*x(i+1) equal to zero. %C A188351 Column 2 of A188358 %H A188351 R. H. Hardin, <a href="/A188351/b188351.txt">Table of n, a(n) for n = 1..200</a> %e A188351 Some solutions for n=4 %e A188351 ..2...-1....1...-1...-2...-1....0....0....0...-1...-1....2....0...-2....1...-1 %e A188351 ..0...-2....2....2....1....1...-1....0...-2...-2...-1....0....0....2....2....1 %e A188351 ..0....1...-2...-1....0....1....0....0....0...-2...-2...-2....2....1....0....1 %e A188351 ..2....0...-2...-2...-2....2....1...-1...-2....2....1....1....0....1...-1....0 %e A188351 ..0...-1....1...-1...-1...-1....0....0....0...-1...-1....2....0....1....2...-1 %K A188351 nonn %O A188351 1,1 %A A188351 _R. H. Hardin_ Mar 28 2011