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 A275541 #6 Jun 02 2025 12:19:12 %S A275541 0,0,1,6,25,126,733,5086,40423,363262,3629743,39920638,479011993, %T A275541 6227066878,87178426333,1307675013118,20922791915023,355687438417918, %U A275541 6402373740187423,121645100594626558,2432902008831369073 %N A275541 (n)! + (n + 1)!!/(n + 1) - 2. %Y A275541 Cf. A006882, A128882. %K A275541 nonn,easy %O A275541 0,4 %A A275541 _Olivier Gérard_, Aug 01 2016