OFFSET
1,1
COMMENTS
The sequence contains a subset of squares {36, 49, 81, 100, 121, 144, 169, 225, 256, ...}.
If n is a term of this sequence and n is nonsquare, then n must be a prime or twice a square. Additionally, if n is in this sequence, then A001221(n) <= 2. - Altug Alkan, Jul 17 2016
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
EXAMPLE
a(6)= 36 with 8 divisors: {1, 2, 3, 4, 6, 9, 12, 18, 36}
and the sum of the divisors is 91 = 7*13 (semiprime).
MAPLE
with(numtheory):for k from 1 to 1600 do:if bigomega(sigma(k))=2 then printf(`%d, `, k): else fi:od:
MATHEMATICA
Select[Range@ 1500, PrimeOmega@ DivisorSigma[1, #] == 2 &] (* Michael De Vlieger, Jul 17 2016 *)
KEYWORD
nonn
AUTHOR
Michel Lagneau, Aug 20 2011
STATUS
approved