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.

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, 35, 33, 37, 39, 37, 41, 31, 43, 17, 45, 43, 47, 9, 49, 47, 51, 53, 55, 53, 57, 47, 59, 57, 61, 47, 63, 65, 63, 67, 57, 69, 67, 71, 73, 23, 75, 73, 77, 43, 79, 77, 81

Offset: 1

Author

Michel Lagneau, Oct 07 2014

Keywords

• nonn

Comments

a(n) = A134407(n) - least prime divisor of A134406(n).

Examples

			a(1) = 1 because the first composite is 3^2+1 = 2*5 and 3-2 = 1.
		

Links

• Michel Lagneau, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A002496, A020639, A089120, A134406, A134407.

Programs

Maple

with(numtheory):
   for n from 1 to 200 do:
    p:=n^2+1:x:=factorset(p):d:=n-x[1]:
    if type(p,prime)=false
    then
    printf(`%d, `,d):
    else
    fi:
   od: