cp's OEIS Frontend

A248511 Difference between k and the least prime factor of k^2+1 where k is the n-th number with k^2+1 composite.

%I A248511 #13 Nov 02 2014 00:42:05
%S A248511 1,3,5,3,7,9,7,11,13,15,13,17,19,17,21,23,25,23,27,13,29,27,31,21,33,
%T A248511 35,33,37,39,37,41,31,43,17,45,43,47,9,49,47,51,53,55,53,57,47,59,57,
%U A248511 61,47,63,65,63,67,57,69,67,71,73,23,75,73,77,43,79,77,81
%N A248511 Difference between k and the least prime factor of k^2+1 where k is the n-th number with k^2+1 composite.
%C A248511 a(n) = A134407(n) - least prime divisor of A134406(n).
%H A248511 Michel Lagneau, <a href="/A248511/b248511.txt">Table of n, a(n) for n = 1..10000</a>
%e A248511 a(1) = 1 because the first composite is 3^2+1 = 2*5 and 3-2 = 1.
%p A248511 with(numtheory):
%p A248511    for n from 1 to 200 do:
%p A248511     p:=n^2+1:x:=factorset(p):d:=n-x[1]:
%p A248511     if type(p,prime)=false
%p A248511     then
%p A248511     printf(`%d, `,d):
%p A248511     else
%p A248511     fi:
%p A248511    od:
%Y A248511 Cf. A002496, A020639, A089120, A134406, A134407.
%K A248511 nonn
%O A248511 1,2
%A A248511 _Michel Lagneau_, Oct 07 2014