Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #24 Jan 25 2024 12:18:52
%S 0,1,2,3,3,4,6,7,8,6,5,9,8,8,14,9,16,17,14,14,16,21,11,19,14,24,25,15,
%T 18,11,28,12,8,19,16,22,35,31,36,25,31,16,40,30,23,41,39,35,10,32,43,
%U 38,24,41,19,35,23,55,54,24,53,50,57,62,48,36,64,21,45,54
%N a(n) = largest k such that prime(n) + A002110(k) is prime.
%C a(n) = n-1 iff a(n) is in A035346 for n > 1.
%F a(n) < n.
%e a(4) = 3 because A002110(3) + prime(4) = A002110(3) + 7 = 37 is prime.
%o (PARI) a(n) = {my(k=1); while(k, if(ispseudoprime(prod(i=1, n-k, prime(i)) + prime(n)), return(n-k)); k++)}
%Y Cf. A000040, A002110, A035346, A100380.
%K nonn
%O 1,3
%A _Altug Alkan_, Dec 01 2015