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 A231885 #21 Jan 10 2022 05:26:08 %S A231885 13,41,131,1429,4861,477638699,4861946401451, %T A231885 5632681584560312734993915705849145099, %U A231885 16435314834665426797069144960762886143367590394939,171069509209912116706646841207804116182333282333996796075729541331934805254423 %N A231885 Primes of the form Catalan(n) - 1. %C A231885 The 22nd term a(22) in the sequence has 862 digits. %C A231885 a(23) has 1134 digits; a(25) has 1413 digits; a(30) has 2046 digits; a(31) has 2348 digits (these are not included in b-file). %H A231885 K. D. Bajpai, <a href="/A231885/b231885.txt">Table of n, a(n) for n = 1..22</a> %e A231885 a(2)= 41: n= 5: (2*n)!/(n!*(n + 1)!)-1= 41 which is prime. %e A231885 a(4)= 1429: n= 8: (2*n)!/(n!*(n + 1)!)-1= 1429 which is prime. %p A231885 KD:= proc() local a; a:= (2*n)!/(n!*(n + 1)!)-1; if isprime(a) then return(a): fi; end: seq(KD(), n=1..150); %t A231885 Select[CatalanNumber[Range[200]]-1,PrimeQ] (* _Harvey P. Dale_, Dec 21 2019 *) %Y A231885 Cf. A000108 (Catalan numbers). %Y A231885 Cf. A053427 (numbers n : Catalan(n)-1 is prime). %Y A231885 Cf. A053429 (numbers n such that Catalan(n)+1 is prime). %Y A231885 Cf. A230061 (primes of the form Catalan(n)+1). %K A231885 nonn %O A231885 1,1 %A A231885 _K. D. Bajpai_, Nov 21 2013