OFFSET
1,2
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
sigma(3)=2^2, sigma(7)=2^3, sigma(21)=2^5, sigma(66)=2^4*3^2.
MAPLE
emin := proc(n::posint) local L; if n>1 then L:=ifactors(n)[2]; L:=map(z->z[2], L); min(L) else 0 fi end: L:=[]: for w to 1 do for n from 1 to 144 do sn:=numtheory[sigma](n); if emin(sn)>1 then L:=[op(L), n]; print(n, ifactor(n), sn, ifactor(sn)) fi; od; od; L;
MATHEMATICA
sigmaPowerQ[1] = True; sigmaPowerQ[n_] := Min@FactorInteger[DivisorSigma[1, n]][[;; , 2]] > 1; Select[Range[1000], sigmaPowerQ] (* Amiram Eldar, Sep 08 2019 *)
PROG
(PARI) isok(n) = ispowerful(sigma(n)); \\ Michel Marcus, Sep 08 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Walter Kehowski, Aug 10 2010
EXTENSIONS
a(1) and more terms from Amiram Eldar, Sep 08 2019
STATUS
approved