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 A258731 #8 Jan 26 2018 08:07:09 %S A258731 16,60,190,512,1212,2592,5115,9460,16588,27820,44928,70240,106760, %T A258731 158304,229653,326724,456760,628540,852610,1141536,1510180,1976000, %U A258731 2559375,3283956,4177044,5269996,6598660,8203840,10131792,12434752,15171497 %N A258731 Number of length n+1 0..3 arrays with at most one downstep in every n consecutive neighbor pairs. %C A258731 Row 1 of A258730. %H A258731 R. H. Hardin, <a href="/A258731/b258731.txt">Table of n, a(n) for n = 1..210</a> %F A258731 Empirical: a(n) = (1/5040)*n^7 + (1/144)*n^6 + (73/720)*n^5 + (91/144)*n^4 + (179/90)*n^3 + (139/36)*n^2 + (568/105)*n + 4. %F A258731 Empirical g.f.: x*(2 - x)*(8 - 30*x + 64*x^2 - 80*x^3 + 58*x^4 - 23*x^5 + 4*x^6) / (1 - x)^8. - _Colin Barker_, Jan 26 2018 %e A258731 Some solutions for n=4: %e A258731 ..3....1....0....1....0....0....2....1....3....0....0....1....1....1....3....3 %e A258731 ..3....3....1....2....0....0....3....2....0....2....2....2....1....1....0....3 %e A258731 ..2....3....0....2....0....0....1....0....0....0....2....3....2....1....1....1 %e A258731 ..2....1....3....2....2....2....2....1....1....0....3....1....3....3....1....3 %e A258731 ..2....1....3....2....1....2....3....1....1....1....3....2....3....0....2....3 %Y A258731 Cf. A258730. %K A258731 nonn %O A258731 1,1 %A A258731 _R. H. Hardin_, Jun 08 2015