A007340 - OEIS

A007340

Numbers whose divisors' harmonic and arithmetic means are both integers.
(Formerly M4299)

1, 6, 140, 270, 672, 1638, 2970, 6200, 8190, 18600, 18620, 27846, 30240, 32760, 55860, 105664, 117800, 167400, 173600, 237510, 242060, 332640, 360360, 539400, 695520, 726180, 753480, 1089270, 1421280, 1539720, 2229500, 2290260, 2457000

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Intersection of A001599 and A003601.

The following are also in A046985: 1, 6, 672, 30240, 32760. Also contains multiply perfect (A007691) numbers. - Labos Elemer

The numbers whose average divisor is also a divisor. Ore's harmonic numbers A001599 without the numbers A046999. - Thomas Ordowski, Oct 26 2014, Apr 17 2022

Harmonic numbers k whose harmonic mean of divisors (A001600) is also a divisor of k. - Amiram Eldar, Apr 19 2022

REFERENCES

G. L. Cohen, personal communication.

Richard K. Guy, Unsolved Problems in Number Theory, 3rd Edition, Springer, 2004, Section B2, pp. 74-84.

N. J. A. Sloane, Illustration for sequence M4299 (=A007340) in The Encyclopedia of Integer Sequences (with Simon Plouffe), Academic Press, 1995.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

D. Wells, Curious and interesting numbers, Penguin Books, p. 124.

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..847

G. L. Cohen, Email to N. J. A. Sloane, Apr. 1994

T. Goto and S. Shibata, All numbers whose positive divisors have integral harmonic mean up to 300, Math. Comput. 73 (2004), 475-491.

Hisanori Mishima, Factorizations of many number sequences

Oystein Ore, On the averages of the divisors of a number, Amer. Math. Monthly, 55 (1948), 615-619.

FORMULA

a = Sigma(1, x)/Sigma(0, x) integer and b = x/a also.

EXAMPLE

x = 270: Sigma(0, 270) = 16, Sigma(1, 270) = 720; average divisor a = 720/16 = 45 and integer 45 divides x, x/a = 270/45 = 6, but 270 is not in A007691.

MAPLE

filter:= proc(n)

uses numtheory;

local a;

a:= sigma(n)/sigma[0](n);

type(a, integer) and type(n/a, integer);

end proc:

select(filter, [$1..2500000]); # Robert Israel, Oct 26 2014

MATHEMATICA

Do[ a = DivisorSigma[0, n]/ DivisorSigma[1, n]; If[IntegerQ[n*a] && IntegerQ[1/a], Print[n]], {n, 1, 2500000}] (* Labos Elemer *)

ahmQ[n_] := Module[{dn = Divisors[n]}, IntegerQ[Mean[dn]] && IntegerQ[HarmonicMean[dn]]]; Select[Range[2500000], ahmQ] (* Harvey P. Dale, Nov 16 2011 *)

PROG

(Haskell)

a007340 n = a007340_list !! (n-1)

a007340_list = filter ((== 0) . a054025) a001599_list

-- Reinhard Zumkeller, Dec 31 2013

(PARI) is(n)=my(d=divisors(n), s=vecsum(d)); s%#d==0 && #d*n%s==0 \\ Charles R Greathouse IV, Feb 07 2017

CROSSREFS

Intersection of A003601 and A001599.

Different from A090945.

Cf. A001600, A007691, A046985-A046987, A046999, A054025.

Sequence in context: A288557 A288565 A090944 * A335318 A342358 A122483

Adjacent sequences: A007337 A007338 A007339 * A007341 A007342 A007343