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A180098 Sigma(A180097(n)), sum of divisors of A180097(n), numbers n such that sigma(n) is powerful. 3
1, 4, 8, 32, 36, 72, 32, 72, 72, 72, 144, 144, 72, 121, 108, 128, 144, 216, 108, 216, 144, 144, 128, 288, 216, 288, 392, 216, 288, 324, 216, 200, 576, 288, 324, 256, 432, 288, 432, 288, 432, 324, 576, 392, 576, 648, 432, 576, 864, 400, 576, 432, 576, 784, 432 (list; graph; refs; listen; history; text; internal format)
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; map(numtheory[sigma], L);

MATHEMATICA

sigmaPowerQ[1] = True; sigmaPowerQ[n_] := Min@FactorInteger[DivisorSigma[1, n]][[;; , 2]] > 1; DivisorSigma[1, #] & /@ Select[Range[400], sigmaPowerQ] (* Amiram Eldar, Sep 08 2019 *)

PROG

(PARI) lista(nn) = {for (n=1, nn, if (ispowerful(s=sigma(n)), print1(s, ", ")); ); } \\ Michel Marcus, Sep 08 2019

CROSSREFS

Cf. A001694, A000203, A180090, A180097.

Sequence in context: A094502 A162459 A129195 * A124143 A173617 A034041

Adjacent sequences:  A180095 A180096 A180097 * A180099 A180100 A180101

KEYWORD

easy,nonn

AUTHOR

Walter Kehowski, Aug 10 2010

EXTENSIONS

a(1) and more terms from Amiram Eldar, Sep 08 2019

STATUS

approved

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Last modified May 12 11:55 EDT 2021. Contains 343821 sequences. (Running on oeis4.)