A206449 Values of sigma(p) of primes p such that sigma(p) = sigma(c) has a solution for some composite number c.

12, 18, 24, 32, 42, 48, 54, 60, 72, 80, 84, 90, 98, 104, 108, 114, 128, 132, 140, 152, 168, 180, 182, 192, 224, 228, 234, 240, 252, 264, 270, 272, 294, 308, 312, 360, 384, 390, 420, 432, 434, 440, 444, 450, 468, 480, 492, 504, 522, 558, 570, 572, 588, 594, 600

Offset: 1

Author

Jaroslav Krizek, Feb 07 2012

Keywords

• nonn

Comments

Corresponding values of sigma(p) of primes p from A158913.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000
• Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Crossrefs

Cf. A000203, A158913 (primes p such that there is a composite c with sigma(p)=sigma(c)), A206448 (values of sigma(c) of composite numbers c such that sigma(c) = sigma(d) has a solution for any other composite number d).

Programs

Mathematica

Union@ Select[ DivisorSigma[1, Select[ Range@ 100, !PrimeQ@# &]], PrimeQ[# - 1] &] (* Robert G. Wilson v, Feb 09 2012 *)

PARI

list(lim) = forprime(p = 1, lim, if(invsigmaNum(p+1) > 1, print1(p+1, ", "))); \\ Amiram Eldar, Dec 16 2024, using Max Alekseyev's invphi.gp

Formula

a(n) = A158913(n) + 1.