A387153 - OEIS

A387153

Squarefree 3-abundant numbers: squarefree numbers k such that A000203(k) > 3*k.

30030, 39270, 43890, 46410, 51870, 53130, 62790, 66990, 67830, 71610, 79170, 82110, 84630, 85470, 91770, 94710, 99330, 101010, 103530, 108570, 111930, 117390, 122430, 128310, 136290, 140910, 144690, 154770, 161070, 164010, 166530, 168630, 182490, 191730, 205590

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OFFSET

1,1

COMMENTS

First differs from A067885 at n = 11: A067885(11) = 72930 is not a term of this sequence. a(59) = 510510 is the least term of this sequence that is not in A067885.

Subsequence of A285615 and first differs from it at n = 51: A285615(51) = 390390 is not a term of this sequence.

This sequence is not the same as the sequence of numbers k such that A048250(k) > 3*k which includes all the terms of this sequence but also nonsquarefree numbers, the least of them is 2*A002110(52) = A088860(52) = 2.1248...*10^96.

The least odd term is A002110(17)/2 = 961380175077106319535, the least term that is not divisible by 3 is a(5607800) = 66853496710, and the least term that is coprime to 6 is A002110(52)/6 = 1.7706...*10^95.

If k is a term and m is a squarefree number coprime to k, then k*m is also a term.

The numbers of terms not exceeding 10^k, for k = 5, 6, ..., are 17, 95, 795, 8162, 86331, 854164, 8372782, ... . Apparently, the asymptotic density of this sequence exists and equals 0.00008... .

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

A001221(a(n)) >= 6.

EXAMPLE

30030 = 2 * 3 * 5 * 7 * 11 * 13 is a term since it is squarefree, and sigma(30030) = 96768 > 3*30030 = 90090.

MATHEMATICA

q[k_] := Module[{f = FactorInteger[k]}, Max[f[[;; , 2]]] == 1 && Times @@ (1 + f[[;; , 1]]) > 3*k]; Select[Range[2*10^5], q]

PROG

(PARI) isok(k) = {my(f = factor(k)); issquarefree(f) && vecprod(apply(x -> x+1, f[, 1])) > 3*k; }

CROSSREFS

Intersection of A005117 and A068403.