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 A255663 #6 Jul 23 2025 15:09:23 %S A255663 256,1024,4048,14192,42653,112196,265430,577464,1174730,2262312, %T A255663 4162792,7370432,12625472,21014456,34103820,54115508,84155145, %U A255663 128505312,192998760,285488992,416438559,599648684,853157474,1200338032,1671232268 %N A255663 Number of length n+3 0..3 arrays with at most two downsteps in every n consecutive neighbor pairs. %C A255663 Row 3 of A255660 %H A255663 R. H. Hardin, <a href="/A255663/b255663.txt">Table of n, a(n) for n = 1..210</a> %F A255663 Empirical: a(n) = (1/39916800)*n^11 + (1/403200)*n^10 + (1/9072)*n^9 + (109/40320)*n^8 + (63421/1209600)*n^7 + (39319/57600)*n^6 + (1888387/362880)*n^5 + (24707/2240)*n^4 + (4099087/453600)*n^3 + (33487/525)*n^2 + (950843/6930)*n + 28 for n>1 %e A255663 Some solutions for n=4 %e A255663 ..3....3....0....0....1....1....2....0....2....3....1....0....3....1....3....3 %e A255663 ..1....1....0....0....0....0....2....1....2....2....2....0....0....0....3....2 %e A255663 ..3....2....3....1....3....1....2....3....0....2....1....2....3....1....0....0 %e A255663 ..0....0....2....1....3....2....2....3....0....1....1....1....3....1....0....0 %e A255663 ..0....2....2....3....3....1....2....3....3....3....0....3....1....3....1....2 %e A255663 ..0....3....2....1....2....1....0....1....1....3....1....3....0....1....1....2 %e A255663 ..3....1....2....3....3....2....1....2....0....1....2....3....1....1....2....2 %Y A255663 Cf. A255660 %K A255663 nonn %O A255663 1,1 %A A255663 _R. H. Hardin_, Mar 01 2015