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 A255664 #6 Jul 23 2025 15:09:30 %S A255664 1024,4096,16128,54560,155144,385738,864924,1788660,3467296,6376160, %T A255664 11223728,19042353,31307660,50094036,78275176,119780409,179919544, %U A255664 265791268,386792720,555250782,787198896,1103326862,1530135124 %N A255664 Number of length n+4 0..3 arrays with at most two downsteps in every n consecutive neighbor pairs. %C A255664 Row 4 of A255660 %H A255664 R. H. Hardin, <a href="/A255664/b255664.txt">Table of n, a(n) for n = 1..210</a> %F A255664 Empirical: a(n) = (1/39916800)*n^11 + (1/362880)*n^10 + (11/80640)*n^9 + (461/120960)*n^8 + (141551/1209600)*n^7 + (36127/17280)*n^6 + (14049449/725760)*n^5 + (18041057/362880)*n^4 + (35402863/302400)*n^3 + (1423187/10080)*n^2 + (1020871/3080)*n + 163 for n>2 %e A255664 Some solutions for n=4 %e A255664 ..2....2....0....1....2....1....3....2....3....3....2....3....0....1....0....3 %e A255664 ..2....3....1....0....1....0....0....0....1....2....1....1....0....3....2....3 %e A255664 ..2....0....0....1....0....0....3....1....1....0....0....2....3....1....3....3 %e A255664 ..3....1....2....2....2....2....0....0....1....0....0....1....0....0....2....2 %e A255664 ..3....0....2....1....3....1....3....0....2....1....1....2....0....0....1....0 %e A255664 ..0....1....2....0....0....3....0....2....1....2....0....3....0....3....2....1 %e A255664 ..3....1....2....0....3....0....1....3....0....0....2....2....1....1....3....3 %e A255664 ..1....1....3....3....2....0....1....2....3....1....2....2....0....1....0....2 %Y A255664 Cf. A255660 %K A255664 nonn %O A255664 1,1 %A A255664 _R. H. Hardin_, Mar 01 2015