OFFSET
1,1
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..10000
FORMULA
Equals { m in A001694 : d(m^2) mod 3 = 0 }.
Sum_{n>=1} 1/a(n) = zeta(2)*zeta(3)/zeta(6) - 5*zeta(3)/(2*zeta(2)) = 0.1166890133... . - Amiram Eldar, Jun 28 2022
EXAMPLE
MATHEMATICA
With[{nn = 10800}, Select[Union@ Flatten@ Table[a^2*b^3, {b, nn^(1/3)}, {a, Sqrt[nn/b^3]}], Mod[DivisorSigma[0, #^2], 3] == 0 &]]
PROG
(PARI) isok(m) = ispowerful(m) && !(numdiv(m^2) % 3); \\ Michel Marcus, Jun 27 2022
(Python)
from sympy import divisor_count as d, factorint as f
def ok(k): return k > 1 and min(f(k).values()) > 1 and d(k*k)%3 == 0
print([k for k in range(11000) if ok(k)]) # Michael S. Branicky, Jun 28 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Michael De Vlieger, Jun 21 2022
STATUS
approved