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A180098
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Sigma(A180097(n)), sum of divisors of A180097(n), numbers n such that sigma(n) is powerful.
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3
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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
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OFFSET
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1,2
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LINKS
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EXAMPLE
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Sigma(3)=2^2, sigma(7)=2^3, sigma(21)=2^5, sigma(66)=2^4*3^2.
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MAPLE
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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);
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MATHEMATICA
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sigmaPowerQ[1] = True; sigmaPowerQ[n_] := Min@FactorInteger[DivisorSigma[1, n]][[;; , 2]] > 1; DivisorSigma[1, #] & /@ Select[Range[400], sigmaPowerQ] (* Amiram Eldar, Sep 08 2019 *)
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PROG
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(PARI) lista(nn) = {for (n=1, nn, if (ispowerful(s=sigma(n)), print1(s, ", ")); ); } \\ Michel Marcus, Sep 08 2019
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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