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 A228215 #6 Jun 02 2025 08:36:44 %S A228215 21,271,2731,23351,176851,1225631,8006491,50556551,313882531, %T A228215 1932641711,11839990891,72260648471,439667406451,2668522016831, %U A228215 16163719991611,97745259402791,590286253682371,3560791008422351,21460113482174731 %N A228215 Number of second differences of arrays of length n+2 of numbers in 0..5. %C A228215 Column 5 of A228218 %H A228215 R. H. Hardin, <a href="/A228215/b228215.txt">Table of n, a(n) for n = 1..189</a> %F A228215 Empirical: a(n) = 11*a(n-1) -30*a(n-2) for n>11 %e A228215 Some solutions for n=4 %e A228215 ..1....2...-1...-7...-4..-10...-6....1....2....8...-3....2...-6...-3....5....6 %e A228215 .-4....2...-3....5....0....8....5....2...-5...-2....1...-2....8....0...-3...-9 %e A228215 ..1...-2...-1...-1....7...-3....2....0....4...-3....4....6...-1....3...-1....5 %e A228215 ..5....3....6....2...-5...-2...-3....5....3....5...-8...-5...-5...-1....6....3 %K A228215 nonn %O A228215 1,1 %A A228215 _R. H. Hardin_ Aug 16 2013