A206448 -id:A206448 - cp's OEIS frontend

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

Jaroslav Krizek, Feb 07 2012

nonn

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

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

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).

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

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

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

cp's OEIS Frontend

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

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