OFFSET
1,2
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
KEYWORD
nonn
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
More terms from Jinyuan Wang, Jun 18 2021
STATUS
approved