A220423 - OEIS

A220423

Cubefree products of primorials (A002110).

1, 2, 4, 6, 12, 30, 36, 60, 180, 210, 420, 900, 1260, 2310, 4620, 6300, 13860, 30030, 44100, 60060, 69300, 180180, 485100, 510510, 900900, 1021020, 3063060, 5336100, 6306300, 9699690, 15315300, 19399380, 58198140, 69369300, 107207100, 223092870, 290990700

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OFFSET

1,2

COMMENTS

Suggested by a comment of Charles R Greathouse IV in A220264.

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

FORMULA

A212793(a(n)) = 1; A051903(a(n)) < 3.

A001221(a(n)) <= A001222(a(n)) <= 2*A001221(a(n)).

A006530(a(n)) = A000040(A001221(a(n))).

Sum_{n>=1} 1/a(n) = (S(1)^2 + S(2))/2 = 2.093360845965235020766040..., where S(k) = Sum_{n>=0} 1/(A002110(n))^k (S(1) = 1 + A064648). - Amiram Eldar, Sep 24 2023

PROG

(Haskell)

import Data.Set (deleteFindMin, empty, fromList, union)

import qualified Data.Set as Set (null)

a220423 n = a220423_list !! (n-1)

a220423_list = f (splitAt 1 a002110_list) empty where

f (us'@(u:_), vs'@(v:vs)) s

| Set.null s || m > u

= f (v:us', vs) (s `union` (fromList $ map (* u) us'))

| otherwise = m : f (us', vs') s'

where (m, s') = deleteFindMin s

CROSSREFS