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

1,1

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).

Formula

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

Mathematica

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

Prog

(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

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).

Sequence in context: A258914 A241646 A181941 * A241852 A113756 A055482

Adjacent sequences: A206446 A206447 A206448 * A206450 A206451 A206452

Keyword

nonn

Author

Jaroslav Krizek, Feb 07 2012

Status

approved