A158913 - OEIS

A158913

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

11, 17, 23, 31, 41, 47, 53, 59, 71, 79, 83, 89, 97, 103, 107, 113, 127, 131, 139, 151, 167, 179, 181, 191, 223, 227, 233, 239, 251, 263, 269, 271, 293, 307, 311, 359, 383, 389, 419, 431, 433, 439, 443, 449, 467, 479, 491, 503, 521, 557, 569, 571, 587, 593, 599

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

See A158914 for the sequence for sigma_2.

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

MATHEMATICA

tp=DivisorSigma[1, Select[Range[1000], PrimeQ]]; tc=DivisorSigma[1, Select[Range[1000], !PrimeQ[ # ]&]]; Intersection[tp, tc]-1

PROG

(Sage) [sigma(n)-1 for n in (2..600) if is_prime(sigma(n)-1) and n<sigma(n)-1<600] # Giuseppe Coppoletta, Dec 22 2014