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

1,1

Comments

Supersequence of A247789.

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

Links

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

Example

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

Prog

(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

Crossrefs

Cf. A000203, A008438, A247789, A247837.

Sequence in context: A305643 A316919 A099974 * A336097 A187894 A214591

Adjacent sequences: A247817 A247818 A247819 * A247821 A247822 A247823

Keyword

nonn

Author

Jaroslav Krizek, Sep 24 2014

Status

approved