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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
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

LINKS

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

EXAMPLE

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

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:

CROSSREFS

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

Sequence in context: A029602 A210041 A240499 * A210191 A110465 A328915

Adjacent sequences:  A248508 A248509 A248510 * A248512 A248513 A248514

KEYWORD

nonn

AUTHOR

Michel Lagneau, Oct 07 2014

STATUS

approved

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Last modified November 22 08:46 EST 2019. Contains 329389 sequences. (Running on oeis4.)