OFFSET
1,2
COMMENTS
Odd refactorable numbers are always squares.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..1001
S. Colton, Refactorable Numbers - A Machine Invention, J. Integer Sequences, Vol. 2, 1999, #2.
FORMULA
a(n) = sqrt(A036896(n)). - Amiram Eldar, Jul 02 2019
EXAMPLE
15^2 is refactorable because 225 has 9 divisors and 9 divides 225.
MATHEMATICA
Select[Range[1, 1000, 2], Divisible[#^2, DivisorSigma[0, #^2]] &] (* Amiram Eldar, Jul 02 2019 *)
PROG
(PARI) isrefac(n) = ! (n % numdiv(n));
lista(nn) = {forstep (n = 1, nn, 2, if (isrefac(n), print1(sqrtint(n), ", ")); ); } \\ Michel Marcus, Aug 31 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Simon Colton (simonco(AT)cs.york.ac.uk)
STATUS
approved