A318807 Numbers whose sum of squarefree divisors and sum of nonsquarefree divisors are both a perfect square.

1, 3, 9, 22, 27, 66, 70, 88, 94, 115, 119, 170, 198, 210, 214, 217, 264, 265, 280, 282, 310, 322, 345, 357, 376, 382, 385, 497, 510, 517, 527, 594, 630, 642, 651, 679, 680, 710, 729, 742, 745, 782, 795, 840, 846, 856, 862, 889, 930, 935, 966, 970, 1035, 1066

Offset

1,2

Comments

Let s be the sum of the squarefree divisors of a number m. The sequence lists the numbers m such that s and sigma(m) - s are both a perfect square.

Or numbers m such that A048250(m) and A162296(m) are perfect squares.

The corresponding pairs of squares (s, sigma(m) - s) are (1, 0), (4, 0), (4, 9), (36, 0), (4, 36), (144, 0), (144, 0), (36, 144), (144, 0), (144, 0), (144, 0), (324, 0), (144, 324), ...

The subsequence b(n) where s and sigma(m) - s are strictly positive begins with 9, 27, 88, 198, 264, 280, 376, 594, 630, ... b(n) is not squarefree (subsequence of A013929).

The subsequence c(n) where the ratio r = (sigma(a(n)) - s)/s is integer begins with 27, 88, 264, 280, 376, 594, 680, 840, 856, 1128, 1240, ... and the corresponding r are 3^2, 2^2, 2^2, 2^2, 2^2, 3^2, 2^2, 2^2, 2^2, 2^2, 2^2, 2^2, 2^2, 5^2, 3^2, 2^2, 7^2, 3^2, 2^2, 11^2, ... It is conjectured that r belongs to A001248.

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

27 is in the sequence because A048250(27) = 4 and A162296(27) = 36 are both a perfect square.

Maple

filter:= proc(n) local F, SF, NSF, t;
  F:= ifactors(n)[2];
  SF:= mul(1+t[1], t=F);
  if not issqr(SF) then return false fi;
  NSF:= mul((1-t[1]^(1+t[2]))/(1-t[1]), t=F) - SF;
  issqr(NSF);
end proc:
select(filter, [$1..2000]); # Robert Israel, Sep 05 2018

Mathematica

lst={}; Do[If[IntegerQ[Sqrt[Total[Select[Divisors[n], SquareFreeQ]]]]&&IntegerQ[Sqrt[DivisorSigma[1, n]-Total[Select[Divisors[n], SquareFreeQ]]]], AppendTo[lst, n]], {n, 1100}]; lst

Prog

(PARI) isok(n) = {my(sd=sumdiv(n, d, issquarefree(d)*d)); issquare(sd) && issquare(sigma(n) - sd); } \\ Michel Marcus, Sep 04 2018

Crossrefs

Cf. A000040, A001248, A013929, A048250, A162296.

Sequence in context: A007647 A247182 A363967 * A063586 A131477 A002128

Adjacent sequences: A318804 A318805 A318806 * A318808 A318809 A318810

Keyword

nonn

Author

Michel Lagneau, Sep 04 2018

Status

approved