OFFSET
1,2
COMMENTS
If m and n are coprime members of the sequence, then m*n is also a member. - Robert Israel, May 10 2018
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..1000
Frits Beukers, Florian Luca and Frans Oort, Power Values of Divisor Sums, The American Mathematical Monthly, Vol. 119, No. 5 (May 2012), pp. 373-380.
FORMULA
sigma(a(n)) = x^7, where the initial values of x are 1, 2, 4, 6 (48 times), ...
EXAMPLE
Divisors(11811) = {1,3,31,93,127,381,3937,11811} and sigma(11811) = 16384 = 4^7.
MAPLE
filter:= n -> type(map(t -> t[2]/7, ifactors(numtheory:-sigma(n))[2]), list(integer)):
select(filter, [$1..21*10^4]); # Robert Israel, May 09 2018
MATHEMATICA
Select[Range[210000], IntegerQ[Surd[DivisorSigma[1, #], 7]]&] (* Harvey P. Dale, Jun 09 2017 *)
PROG
(PARI) isok(n) = ispower(sigma(n), 7); \\ Michel Marcus, Dec 20 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved