A247820 Numbers k such that sigma(2*k-1) is a prime p.

5, 13, 145, 365, 841, 1201, 1741, 2521, 3961, 5101, 7813, 8581, 13945, 14281, 14965, 41761, 42925, 73345, 139921, 229165, 245701, 265721, 276025, 289561, 298765, 341965, 351961, 353641, 367225, 414961, 595141, 601705, 676285, 697381, 711625, 740545, 942565

Offset: 1

Author

Jaroslav Krizek, Sep 24 2014

Keywords

• nonn

Comments

Supersequence of A247789.

Corresponding values of primes p for a(n) are A008438(a(n)-1) or A247837.

Examples

			Number 13 is in sequence because sigma(2*13-1) = sigma(25) = 31 (prime).
		

Links

• Martin Møller Skarbiniks Pedersen, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000203, A008438, A247789, A247837.

Programs

Magma

[n: n in [1..10000000] | IsPrime(SumOfDivisors(2*n-1))];

PARI

for(n=1,10^7,if(isprime(sigma(2*n-1)),print1(n,", "))) \\ Derek Orr, Sep 25 2014