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 A212777 #8 Jul 21 2018 10:06:01 %S A212777 28,99,350,1238,4379,15490,54793,193821,685609,2425226,8578828, %T A212777 30346157,107344412,379712753,1343169821,4751236703,16806698494, %U A212777 59450861306,210297394895,743891565706,2631390949017,9308101672049,32925839761497 %N A212777 Half the number of 0..3 arrays of length n+2 with second differences nonzero. %C A212777 Column 3 of A212782. %H A212777 R. H. Hardin, <a href="/A212777/b212777.txt">Table of n, a(n) for n = 1..210</a> %F A212777 Empirical: a(n) = 2*a(n-1) + 5*a(n-2) + 3*a(n-3) - 4*a(n-4) - 4*a(n-5). %F A212777 Empirical g.f.: x*(28 + 43*x + 12*x^2 - 41*x^3 - 32*x^4) / (1 - 2*x - 5*x^2 - 3*x^3 + 4*x^4 + 4*x^5). - _Colin Barker_, Jul 21 2018 %e A212777 Some solutions for n=5: %e A212777 ..3....0....3....3....3....3....2....2....3....3....1....3....1....3....0....0 %e A212777 ..1....3....1....2....3....3....2....2....2....3....0....1....0....1....1....1 %e A212777 ..3....0....0....3....1....1....0....0....3....0....1....3....2....1....3....0 %e A212777 ..1....0....1....0....2....0....3....1....3....3....3....2....2....3....1....3 %e A212777 ..1....3....0....1....2....0....0....3....2....3....2....3....1....3....0....3 %e A212777 ..2....3....1....0....0....1....2....2....2....0....0....0....2....0....0....1 %e A212777 ..1....1....1....1....1....0....3....2....0....1....3....2....2....3....1....3 %Y A212777 Cf. A212782. %K A212777 nonn %O A212777 1,1 %A A212777 _R. H. Hardin_, May 27 2012