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 A255661 #7 Jan 21 2018 09:37:06 %S A255661 16,64,255,968,3340,10320,28722,72920,171106,375388,777452,1532064, %T A255661 2891360,5253680,9231663,15745452,26148180,42392440,67248205, %U A255661 104584680,159730860,239932160,354923400,517641696,745106454,1059497716,1489468594 %N A255661 Number of length n+1 0..3 arrays with at most two downsteps in every n consecutive neighbor pairs. %C A255661 Row 1 of A255660. %H A255661 R. H. Hardin, <a href="/A255661/b255661.txt">Table of n, a(n) for n = 1..210</a> %F A255661 Empirical: a(n) = (1/39916800)*n^11 + (1/518400)*n^10 + (1/15120)*n^9 + (137/120960)*n^8 + (12461/1209600)*n^7 + (8251/172800)*n^6 + (56011/362880)*n^5 + (14791/25920)*n^4 + (278149/151200)*n^3 + (8149/2100)*n^2 + (2539/462)*n + 4. %F A255661 Empirical g.f.: x*(16 - 128*x + 543*x^2 - 1388*x^3 + 2394*x^4 - 2964*x^5 + 2683*x^6 - 1760*x^7 + 814*x^8 - 252*x^9 + 47*x^10 - 4*x^11) / (1 - x)^12. - _Colin Barker_, Jan 21 2018 %e A255661 Some solutions for n=4: %e A255661 ..0....2....1....2....1....1....1....3....2....3....0....0....0....3....1....1 %e A255661 ..2....3....1....2....2....3....1....0....0....0....1....3....2....3....2....3 %e A255661 ..1....0....2....2....0....0....3....0....3....0....1....0....0....0....1....0 %e A255661 ..3....3....3....0....3....0....0....0....1....1....3....3....0....3....1....2 %e A255661 ..3....0....2....3....0....0....0....3....1....0....3....1....2....3....2....0 %Y A255661 Cf. A255660. %K A255661 nonn %O A255661 1,1 %A A255661 _R. H. Hardin_, Mar 01 2015