OFFSET
1,1
COMMENTS
The base-7 representation of a term 7^i + 7^j - 1 has base-7 digital sum = 1 + 6*j == 1 (mod 6).
Numbers m that satisfy m = 7^i + 7^j + 1 are never primes, since the base-7 digital sum of m is 3, and thus, m is divisible by 3.
LINKS
Hieronymus Fischer, Table of n, a(n) for n = 1..40
EXAMPLE
a(1) = 7, since 7 = 7^1 + 7^0 - 1 is prime.
a(2) = 349, since 349 = 7^3 + 7^1 - 1 is prime.
MATHEMATICA
Select[Flatten[Table[7^x+7^y-1, {x, 0, 20}, {y, 0, x-1}]], PrimeQ] (* Harvey P. Dale, Aug 13 2023 *)
PROG
CROSSREFS
KEYWORD
nonn
AUTHOR
Hieronymus Fischer, Apr 14 2014
STATUS
approved