OFFSET
1,1
COMMENTS
If (3^i-7)/2 is prime then x=3^i-7 is a solution of sigma(x)+7 = sigma(x+7). See A063680 and indices resulting in primes are in A063679.
The next term (a(7)) has 118 digits. - Harvey P. Dale, May 10 2014
FORMULA
2*37+7 = 3^4, so 37 is in the sequence.
MATHEMATICA
Select[(#-7)/2&/@(3^Range[2, 250]), PrimeQ] (* Harvey P. Dale, May 10 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Jud McCranie, Jul 29 2001
STATUS
approved