OFFSET
1,1
COMMENTS
The corresponding sequence b(n) = {r} begins with {4, 4, 9, 20, 4, 9, 4, 4, 20, 4, 4, 25, 84, 9, 4, 4, 20, 4, 4, 9, 49, 20, 4, 90, 4, 25, ... }. A majority of numbers of b(n) are perfect squares.
The numbers 2^(2n+1) with k > 0 are in the sequence (A004171).
The numbers prime(n)^3 are in the sequence (A030078).
The numbers 8*prime(n) with n > 1 are in the sequence.
Note that "positive integer", in the definition, eliminates squarefree numbers (A005117) from this sequence. - Michel Marcus, Mar 24 2018
From Robert Israel, Mar 29 2018: (Start)
If n is in the sequence, then so is n*p for any prime p coprime to n.
If m and n are in the sequence and are coprime, then m*n is in the sequence. (End)
The exponentially odd numbers (A268335) that are not squarefree are in the sequence. - Amiram Eldar, Jul 04 2020
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
filter:= proc(n) local S, N; uses numtheory;
S, N:= selectremove(issqrfree, divisors(n));
N <> {} and type(convert(N, `+`)/convert(S, `+`), integer)
end proc:
select(filter, [$1..1000]); # Robert Israel, Mar 29 2018
MATHEMATICA
lst={}; Do[If[DivisorSigma[1, n]-Total[Select[Divisors[n], SquareFreeQ]]>0&&IntegerQ[(DivisorSigma[1, n]-Total[Select[Divisors[n], SquareFreeQ]])/Total[Select[Divisors[n], SquareFreeQ]]], AppendTo[lst, n]], {n, 520}]; lst
rpiQ[n_]:=Module[{d=Divisors[n], sf, ot, ra}, sf=Select[d, SquareFreeQ]; ot=Complement[ d, sf]; ra= Total[ ot]/Total[sf]; ra>0&&IntegerQ[ra]]; Select[Range[600], rpiQ] (* Harvey P. Dale, Mar 19 2019 *)
f[p_, e_] := (p^(e + 1) - 1)/(p^2 - 1); ratio[n_] := Times @@ (f @@@ FactorInteger[n]); Select[Range[2, 520], (r = ratio[#]) > 1 && IntegerQ[r] &] (* Amiram Eldar, Jul 04 2020 *)
PROG
(PARI) isok(n) = my(s = sumdiv(n, d, !issquarefree(d)*d)); s && !(s % (sigma(n) - s)); \\ Michel Marcus, Mar 24 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Mar 23 2018
STATUS
approved