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 A273983 #14 Jun 15 2016 09:23:10 %S A273983 1,31,2745,487935,145769625,65830256415,41892106080825, %T A273983 35736278004165375,39370290736153001625,54420772423242699849375, %U A273983 92234193751998833171261625,188098544080793843475953349375,454418941572893462364414856265625,1283429428883663190972186961851609375 %N A273983 a(n) = ((4*n)!! - (4*n-1)!!)/(4*n+1). %C A273983 Sequence is inspired by A273889. The same argument in A273889 can be used here to prove the expression evaluates to integers. %H A273983 Chai Wah Wu, <a href="/A273983/b273983.txt">Table of n, a(n) for n = 1..202</a> %o A273983 (Python) %o A273983 doublefac=lambda x:1 if x<2 else x*doublefac(x-2) %o A273983 for i in range(200): %o A273983 print(i,(doublefac(4*i)-doublefac(4*i-1))//(4*i+1)) %o A273983 # _Brian Cheung_, Jun 15 2016 %Y A273983 Cf. A273889, A007912. %K A273983 nonn %O A273983 1,2 %A A273983 _Chai Wah Wu_, Jun 05 2016