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 A202121 #7 Jun 02 2025 07:24:02 %S A202121 11,91,587,2833,14063,69573,345241,1713419,8503671,42203951,209464959, %T A202121 1039605903,5159703889,25608300207,127097485317,630802143547, %U A202121 3130756848055,15538372099147,77119055108453,382752363927519 %N A202121 Number of -5..5 arrays of n elements with first, second and third differences also in -5..5. %C A202121 Column 5 of A202124 %H A202121 R. H. Hardin, <a href="/A202121/b202121.txt">Table of n, a(n) for n = 1..210</a> %e A202121 Some solutions for n=5 %e A202121 .-3...-3...-2...-3....0....4....0....0...-4....0...-5....1...-5....3....2...-4 %e A202121 ..0....1....0...-1....2....0....2...-2...-1....1...-5...-3...-4....3....0...-5 %e A202121 ..0....5...-2....2....2....0....3...-3....2....1...-2...-4....0....1...-3...-4 %e A202121 ..2....4...-3....3....0....0....5...-3....4...-2....0....0....2....0...-4...-3 %e A202121 ..5...-1...-5....3....0...-5....3....0....3...-5....4....4....1....2...-1....1 %K A202121 nonn %O A202121 1,1 %A A202121 _R. H. Hardin_ Dec 11 2011