A265109 - 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