

A275674


Numbers n for which sum of divisors of sum of divisors of n is a power of 2.


1



1, 2, 4, 16, 25, 50, 64, 100, 361, 1444, 1600, 4096, 5776, 9025, 36100, 65536, 102400, 262144, 1478656, 1638400, 6553600, 23658496, 36966400, 94633984, 591462400, 1073741824, 2365849600, 26843545600
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OFFSET

1,2


LINKS

Table of n, a(n) for n=1..28.


FORMULA

A051027(a(n)) = 2^k for some k.


EXAMPLE

4 is in this sequence because A051027(4) = 8 = 2^3.


MATHEMATICA

Select[Range[10^6], IntegerQ@ Log2@ Nest[DivisorSigma[1, #] &, #, 2] &] (* Michael De Vlieger, Aug 04 2016 *)


PROG

(MAGMA) [n: n in [1..100000]  NumberOfDivisors(2*DivisorSigma(1, DivisorSigma(1, n))) NumberOfDivisors(DivisorSigma(1, DivisorSigma(1, n))) eq 1];
(PARI) isok(n) = (n==1)  (ispower(sigma(sigma(n)), , &s) && (s==2)); \\ Michel Marcus, Aug 05 2016
(PARI) ispow2(n)=n>>valuation(n, 2)==1
forfactored(n=1, 10^9, if(ispow2(sigma(sigma(n[2]))), print1(n[1]", "))) \\ Charles R Greathouse IV, Aug 19 2017


CROSSREFS

Cf. A051027.
Sequence in context: A239363 A045541 A234639 * A225983 A298080 A292369
Adjacent sequences: A275671 A275672 A275673 * A275675 A275676 A275677


KEYWORD

nonn,more


AUTHOR

JuriStepan Gerasimov, Aug 04 2016


EXTENSIONS

a(22) corrected and a(23)a(24) from Michel Marcus, Aug 05 2016
a(25)a(27) from Charles R Greathouse IV, Aug 19 2017
a(28) from Charles R Greathouse IV, Aug 20 2017


STATUS

approved



