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A350338 Number of nontrivial divisors of n that are the product of up to 3 (not necessarily distinct) primes. 2
0, 1, 1, 2, 1, 3, 1, 3, 2, 3, 1, 5, 1, 3, 3, 3, 1, 5, 1, 5, 3, 3, 1, 6, 2, 3, 3, 5, 1, 7, 1, 3, 3, 3, 3, 7, 1, 3, 3, 6, 1, 7, 1, 5, 5, 3, 1, 6, 2, 5, 3, 5, 1, 6, 3, 6, 3, 3, 1, 10, 1, 3, 5, 3, 3, 7, 1, 5, 3, 7, 1, 8, 1, 3, 5, 5, 3, 7, 1, 6, 3, 3, 1, 10, 3, 3, 3, 6, 1, 10, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,4
COMMENTS
Number of divisors of n of the form p, p^2, p*q, p^3, p^2*q, or p*q*r where p,q,r are primes.
LINKS
FORMULA
a(n) = Sum_{d|n} Sum_{k=1..3} [Omega(d) = k], where [ ] is the Iverson bracket.
MATHEMATICA
a[n_] := DivisorSum[n, 1 &, 0 < PrimeOmega[#] <= 3 &]; Array[a, 100] (* Amiram Eldar, Dec 26 2021 *)
PROG
(PARI) A350338(n) = sumdiv(n, d, (d>1)*(bigomega(d)<=3)); \\ Antti Karttunen, Nov 02 2022
CROSSREFS
Cf. A001222 (Omega), A350331.
Sequence in context: A079167 A304793 A199570 * A239707 A294928 A325770
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Dec 25 2021
STATUS
approved

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Last modified July 13 18:16 EDT 2024. Contains 374285 sequences. (Running on oeis4.)