OFFSET
1,1
COMMENTS
Numbers whose prime factorization has a single exponent that is equal to 4 and all the rest, if they exist, are equal to 2. - Amiram Eldar, Jun 25 2022
LINKS
Hugo Pfoertner, Table of n, a(n) for n = 1..10000
FORMULA
From Amiram Eldar, Jun 25 2022: (Start)
a(n) = A060687(n)^2.
Sum_{n>=1} 1/a(n) = (15/Pi^2) * Sum_{k>=2} (-1)^k * P(2*k) = 0.09603403868516162554..., where P(s) is the prime zeta function. (End)
MATHEMATICA
sdscQ[n_]:=Count[Rest[Divisors[n]], _?(IntegerQ[Surd[#, 3]]&)]==1; Select[ Range[ 250]^2, sdscQ] (* Harvey P. Dale, Aug 18 2020 *)
q[n_] := (e = Sort[FactorInteger[n][[;; , 2]], Greater])[[1]] == 4 && AllTrue[Rest[e], # == 2 &]; Select[Range[200]^2, q] (* Amiram Eldar, Jun 25 2022 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, Oct 25 2018
STATUS
approved