1,1

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

Table of n, a(n) for n=1..6.

2*37+7 = 3^4, so 37 is in the sequence.

Select[(#-7)/2&/@(3^Range[2, 250]), PrimeQ] (* Harvey P. Dale, May 10 2014 *)

Cf. A063679, A063680.

Sequence in context: A262786 A140746 A262646 * A289820 A087020 A238840

Adjacent sequences: A063678 A063679 A063680 * A063682 A063683 A063684

nonn

Jud McCranie, Jul 29 2001

approved