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 A215775 #24 Aug 14 2024 01:51:45
%S A215775 0,1,2,3,4,5,6,7,8,10,13,17,18,24,28,30,39,51,149,197,355,444,623,908,
%T A215775 1084,1125,1577,1748,2584,2628,2702,3973,5046,6048,6058,7123,9706,
%U A215775 14223,17775
%N A215775 Numbers k such that 2*k!! + 1 is a prime.
%C A215775 The corresponding primes are in A215777.
%e A215775 17 is in the sequence because 2*17!! + 1 = 2*34459425 + 1 = 68918851 is prime.
%t A215775 lst={};Do[If[PrimeQ[2*n!!+1],AppendTo[lst,n]],{n,0,1500}];lst
%o A215775 (Python)
%o A215775 from sympy import isprime
%o A215775 def afind(startat=0, limit=10**9):
%o A215775 df, dfm1 = 1, 1
%o A215775 for k in range(startat):
%o A215775 df, dfm1 = (k+1)*dfm1, df
%o A215775 for k in range(startat, limit+1):
%o A215775 if isprime(2*df+1): print(k, end=", ")
%o A215775 df, dfm1 = (k+1)*dfm1, df
%o A215775 afind(limit=625) # _Michael S. Branicky_, May 16 2021
%Y A215775 Cf. A006882, A215777.
%K A215775 nonn,hard,more
%O A215775 1,3
%A A215775 _Michel Lagneau_, Aug 23 2012
%E A215775 Offset changed to 1 by and a(29)-a(31) from _Jinyuan Wang_, May 15 2021
%E A215775 a(32)-a(33) from _Michael S. Branicky_, May 16 2021
%E A215775 a(34)-a(39) from _Michael S. Branicky_, Aug 13 2024