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a(n) is the least prime p such that p*(p+prime(n))-1 is prime with prime(i)=i-th prime.
2

%I #8 Sep 24 2019 02:41:00

%S 2,3,2,2,3,2,2,2,5,2,3,7,3,2,2,2,7,3,2,5,2,5,3,2,2,3,3,7,5,2,2,3,2,2,

%T 5,3,2,5,2,2,5,5,11,2,2,2,3,2,2,2,11,7,5,3,7,3,2,3,2,5,2,3,2,3,3,5,11,

%U 2,3,2,2,29,3,5,2,2,37,2,5,2,11,5,3,3,2,5,5,5,23,2,2,17,2,3,7,2,2,3,2,5,2,2

%N a(n) is the least prime p such that p*(p+prime(n))-1 is prime with prime(i)=i-th prime.

%H Pierre CAMI, <a href="/A143393/b143393.txt">Table of n, a(n) for n = 1..12250</a>

%e 2*(2+2)-1=7 prime so a(1)=2 as prime(1)=2;

%e 2*(2+3)-1=9 composite; 3*(3+3)-1=17 prime so a(2)=3 as prime(2)=3.

%o (PARI) a(n) = {my(pn=prime(n), p=2); while (!isprime(p*(p+pn)-1), p = nextprime(p+1)); p;} \\ _Michel Marcus_, Sep 24 2019

%Y Cf. A143394.

%K nonn

%O 1,1

%A _Pierre CAMI_, Aug 12 2008

%E Name edited by _Michel Marcus_, Sep 24 2019