A158914 Primes p such that there is a composite c with sigma_2(p)=sigma_2(c).

7, 47, 157, 3863

Offset

1,1

Comments

No other terms less than 10^8. The corresponding composite numbers are 6, 40, 136, and 3352. Is this sequence finite? See A158913 for the sequence for sigma_1.

Links

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

Mathematica

tp=DivisorSigma[2, Select[Range[4000], PrimeQ]]; tc=DivisorSigma[2, Select[Range[4000], !PrimeQ[ # ]&]]; Sqrt[Intersection[tp, tc]-1]

Crossrefs

Sequence in context: A348394 A158713 A142185 * A046872 A167860 A152988

Adjacent sequences: A158911 A158912 A158913 * A158915 A158916 A158917

Keyword

more,nonn

Author

T. D. Noe, Mar 30 2009

Status

approved