A187562 - OEIS

%I #10 Jan 29 2019 01:51:34

%S 0,1,3,1,3,1,3,7,3,11,9,3,3,3,3,9,5,3,1,11,9,1,9,5,7,5,3,9,7,11,3,21,

%T 23,1,11,1,3,1,9,3,5,15,5,1,3,9,3,13,3,7,3,5,13,21,9,21,9,21,21,5,3,

%U 21,3,3,9,11,15,1,11,1,3

%N Least k >= 0 such that prime(n)*(prime(n)-k)+1 is prime.

%H Pierre CAMI, <a href="/A187562/b187562.txt">Table of n, a(n) for n = 1..25000</a>

%p A187562 := proc(n) local p,k; p := ithprime(n) ; for k from 0 do if isprime( p*(p-k)+1 ) then return k; end if;  end do: end proc: # _R. J. Mathar_, Mar 13 2011

%t Table[k = 0; While[! PrimeQ[Prime[n]*(Prime[n] - k) + 1], k++]; k, {n, 100}]

%K nonn

%O 1,3

%A _Pierre CAMI_, Mar 13 2011