1,1

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.

Table of n, a(n) for n=1..4.

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

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

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

Sequence in context: A070231 A263049 A167917 * A163081 A096239 A074699

Adjacent sequences: A249758 A249759 A249760 * A249762 A249763 A249764

nonn,more

Jaroslav Krizek, Nov 13 2014

approved