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 A127691 #5 Feb 27 2019 21:41:01 %S A127691 -1,0,0,2,64,2002,69264,2697354,117899520,5740530914,308814720400, %T A127691 18213955960378,1169672907313728,81286160430474066, %U A127691 6080123714344570128,487166244246501489002,41636260100744439399424,3781449867356996500139458,363724112817207189865471248 %N A127691 a(n) = (2*n+1)^n-(2*n-1)^n-(2*n)^n. %C A127691 Theorem: (2*n+1)^n-(2*n-1)^n-(2*n)^n > 0 for n >= 3. %D A127691 D. S. Mitrinovic, Analytic Inequalities, Springer-Verlag, 1970; p. 192, 3.1.13. %K A127691 sign %O A127691 0,4 %A A127691 _N. J. A. Sloane_, Apr 03 2007