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A065149 Composite numbers m such that phi(m)*sigma(m) is divisible by m-1. 1
10, 33, 65, 136, 145, 261, 385, 451, 897, 946, 1281, 1441, 1665, 1729, 2241, 2353, 3585, 5185, 6721, 7201, 8380, 8911, 8961, 11521, 11782, 12673, 12801, 17101, 18241, 20737, 25201, 26625, 26677, 26937, 29697, 29953, 30721, 30889, 32896 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..500

FORMULA

(A000010(m)*A000203(m)) mod (m-1) = 0, m is composite.

EXAMPLE

m=136, phi(136)=64, sigma(136)=270, product=17280, quotient=128; for primes the formula holds.

MAPLE

with(numtheory): select(m->modp(phi(m)*sigma(m), m-1)=0 and not isprime(m), [$2..40000]); # Muniru A Asiru, Jun 18 2018

MATHEMATICA

Do[s=EulerPhi[n]*DivisorSigma[1, n]; If[IntegerQ[s/(n-1)]&&!PrimeQ[n], Print[n]], {n, 1, 100000}]

PROG

(PARI) { n=0; for (m=2, 10^9, s=eulerphi(m)*sigma(m); if (s%(m-1) == 0 && !isprime(m), write("b065149.txt", n++, " ", m); if (n==500, return)) ) } \\ Harry J. Smith, Oct 12 2009

(GAP) Filtered([2..40000], m->Phi(m)*Sigma(m) mod (m-1)=0 and not IsPrime(m)); # Muniru A Asiru, Jun 18 2018

CROSSREFS

Cf. A000010, A000203, A062354, A011257.

Sequence in context: A067878 A067877 A063160 * A299287 A299285 A081437

Adjacent sequences:  A065146 A065147 A065148 * A065150 A065151 A065152

KEYWORD

nonn

AUTHOR

Labos Elemer, Oct 18 2001

EXTENSIONS

Offset changed from 0 to 1 by Harry J. Smith, Oct 12 2009

STATUS

approved

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Last modified January 23 16:36 EST 2020. Contains 331172 sequences. (Running on oeis4.)