OFFSET
1,1
COMMENTS
First differs from A286708 at n = 25.
Number of the form p^i * q^j, where p != q are primes and i,j > 1.
The possible values of the number of the divisors (A000005) of terms in this sequence is any composite number that is not 8 or twice a prime (A264828 \ {1, 8}).
675 = 3^3*5^2 and 676 = 2^2*13^2 are 2 consecutive integers in this sequence. There are no other such pairs below 10^22 (the lesser members of such pairs are terms of A060355).
LINKS
FORMULA
Sum_{n>=1} 1/a(n) = ((Sum_{p prime} (1/(p*(p-1))))^2 - Sum_{p prime} (1/(p^2*(p-1)^2)))/2 = 0.1583860791... .
EXAMPLE
36 is a term since 36 = 2^2 * 3^2.
MATHEMATICA
Select[Range[2500], Length[(e = FactorInteger[#][[;; , 2]])] == 2 && Min[e] > 1 &]
PROG
(PARI) is(n) = {my(f=factor(n)); #f~ == 2 && vecmin(f[, 2]) > 1};
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Jul 03 2022
STATUS
approved