A325022 Harmonic numbers m from A001599 such that m*(m-tau(m))/sigma(m) is not an integer, where k-tau(k) = the number of nondivisors of k (A049820), tau(k) = the number of divisors of k (A000005) and sigma(k) = the sum of the divisors of k (A000203).

140, 270, 1638, 2970, 6200, 8190, 18600, 18620, 27846, 55860, 105664, 117800, 167400, 173600, 237510, 242060, 360360, 539400, 726180, 753480, 950976, 1089270, 1421280, 1539720, 2229500, 2290260, 2457000, 2845800, 4358600, 4713984, 4754880, 5772200, 6051500

Offset

1,1

Comments

Numbers m such that sigma(m) divides m*tau(m) but sigma(m) does not divide m*(m-tau(m)).

Complement of A325021 with respect to A001599.

Links

Table of n, a(n) for n=1..33.

Example

140 is a term because 140*(140-tau(140))/sigma(140) = 140*(140-12)/336 = 160/3.

Mathematica

Select[Range[10^5], And[IntegerQ@ HarmonicMean@ #4, ! IntegerQ[#1 (#1 - #2)/#3]] & @@ Append[{#}~Join~DivisorSigma[{0, 1}, #], Divisors@ #] &] (* Michael De Vlieger, Mar 30 2019 *)

Prog

(Magma) [n: n in [1..1000000] | IsIntegral((NumberOfDivisors(n) * n) / SumOfDivisors(n)) and not IsIntegral(((n-NumberOfDivisors(n)) * n) / SumOfDivisors(n))]
(PARI) isok(m) = my(d=numdiv(m), s=sigma(m)); !frac(m*d/s) && frac(m*(m-d)/s); \\ Michel Marcus, Mar 28 2019
(Python)
from itertools import count, islice
from math import prod
from functools import reduce
from sympy import factorint
def A325022_gen(startvalue=1): # generator of terms >= startvalue
    for n in count(max(startvalue, 1)):
        f = factorint(n)
        s = prod((p**(e+1)-1)//(p-1) for p, e in f.items())
        if n*n%s and not reduce(lambda x, y:x*y%s, (e+1 for e in f.values()), 1)*n%s:
            yield n
A325022_list = list(islice(A325022_gen(), 10)) # Chai Wah Wu, Feb 14 2023

Crossrefs

Cf. A000005, A000203, A001599, A049820, A325020, A325021, A325023, A325024.

Sequence in context: A259718 A090945 A140798 * A337689 A360217 A249554

Adjacent sequences: A325019 A325020 A325021 * A325023 A325024 A325025

Keyword

nonn

Author

Jaroslav Krizek, Mar 28 2019

Status

approved