A249761 Primes p such that there is prime q with sigma(q-1) = p.

3, 7, 31, 131071

Offset: 1

Author

Jaroslav Krizek, Nov 13 2014

Keywords

• nonn
• more

Comments

a(n) = corresponding values of sigma(n) of numbers n from A249760(n); a(n) = A000203(A249760(n)).

Corresponding values of primes q are in A249759.

Conjectures: 1) sequence is finite; 2) (a(n)+3)/2 is a Fermat prime (A019434); 3) a(n) is subsequence of Mersenne primes (A000668); 4) a(n) = 2*A249759(n)-3.

Examples

			Prime 31 is in the sequence because there is prime 17 with sigma(17-1) = sigma(16) = 31.
		

Crossrefs

Cf. A000203, A000668, A019434, A249759, A249760.

Programs

Magma

[a: p in PrimesUpTo(5000000) | IsPrime(a) where a is  SumOfDivisors(p-1)]