OFFSET
1,2
COMMENTS
Nonprime numbers k such that Chowla(k)/phi(k) is an integer.
a(7) > 3.7*10^12. - Giovanni Resta, Jul 14 2013
3^30*13*852977547249259 and 3^36*13*621820631944710643 are also terms. - Giovanni Resta, Nov 14 2019
EXAMPLE
Below 30000000 only 5 composite numbers were found: C = {4,15,900,902,28611063}, Chowla(C) = {2,8,1920,504,17600976}, phi(C) = {2,8,240,504,17600976}, quotient = {1,1,8,1,1}.
MATHEMATICA
s2[x_] := DivisorSigma[1, x]-x-1 e0[x_] := EulerPhi[x] Do[s=s2[n]/e0[n]; If[IntegerQ[s]&&!PrimeQ[n], Print[{n, s2[n], e0[n], s}]], {n, 1, 1000000}]
PROG
(PARI) is(n)=!isprime(n) && Mod(sigma(n), eulerphi(n))==n+1 \\ Charles R Greathouse IV, Jun 06 2013
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Labos Elemer, Apr 18 2002
STATUS
approved