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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 (list; graph; refs; listen; history; text; internal format)
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

Juri-Stepan 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

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Last modified January 20 11:11 EST 2020. Contains 331083 sequences. (Running on oeis4.)