OFFSET
1,1
COMMENTS
Equivalently, numbers r such that 6*r+1 has a nontrivial factor == 1 (mod 6).
These numbers, together with numbers of the form 6*j*k-j-k (A070799) are the numbers s for which 6*s+1 is composite (A046954). If we also add in the numbers of the form 6*j*k+j-k (A046953), we get the numbers t such that 6*t-1 and 6*t+1 do not form a pair of twin primes (A067611).
If N is the set of natural numbers, then the set N-{A070043 U A070799} are the numbers k that make 6*k+1 prime. - Pedro Caceres, Jan 22 2018
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..9000
EXAMPLE
41 = 6*2*3 + 2 + 3. Equivalently, 6*41+1 = (6*2+1)*(6*3+1).
MATHEMATICA
Select[Range[250], MemberQ[Mod[Take[Divisors[6#+1], {2, -2}], 6], 1]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Jon Perry, May 05 2002
EXTENSIONS
Edited by Dean Hickerson and Vladeta Jovovic, May 07 2002
STATUS
approved