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A067683

Numbers k such that phi(k)*sigma(k) + 1 is prime.

1, 8, 9, 10, 12, 15, 20, 28, 30, 32, 35, 36, 42, 45, 50, 54, 58, 60, 70, 80, 92, 93, 95, 100, 110, 114, 122, 123, 124, 125, 130, 132, 142, 143, 145, 152, 155, 162, 165, 168, 169, 171, 172, 174, 175, 176, 178, 180, 183, 185, 186, 195, 198, 200, 204, 209, 212, 216

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

OFFSET

1,2

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

MATHEMATICA

Select[Range[216], PrimeQ[EulerPhi[#]*DivisorSigma[1, #] + 1] &] (* Amiram Eldar, Apr 20 2025 *)

PROG

(PARI) isok(k) = {my(f = factor(k)); isprime(eulerphi(k) * sigma(k) + 1); } \\ Amiram Eldar, Apr 20 2025

CROSSREFS

Cf. A000010 (phi), A000203 (sigma), A062354.

Sequence in context: A209724 A114842 A169928 * A338026 A359881 A384430