login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A325020 Numbers m such that m*(m-tau(m))/sigma(m) is a integer h where k-tau(k) is the number of nondivisors of k (A049820) and sigma(k) is the sum of the divisors of k (A000203). 4
1, 2, 6, 22, 28, 76, 84, 90, 96, 170, 216, 248, 252, 496, 520, 532, 588, 672, 700, 852, 864, 1240, 2176, 2448, 2480, 2812, 3360, 6048, 7392, 7584, 8128, 9120, 11480, 12616, 12768, 13832, 14056, 14720, 15456, 19488, 20536, 21216, 27000, 30240, 31584, 31968 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Even perfect numbers from A000396 are terms.

Corresponding values of integers h: 0, 0, 1, 11, 11, 38, 27, 30, 32, 85, 72, 124, 81, ...

Supersequence of A325021 and A325023.

LINKS

Table of n, a(n) for n=1..46.

EXAMPLE

28 is a term because 28*(28-tau(28))/sigma(28) = 28*(28-6)/56 = 11 (integer).

MATHEMATICA

Select[Range[10^5], IntegerQ[#1 (#1 - #2)/#3] & @@ Prepend[DivisorSigma[{0, 1}, #], #] &] (* Michael De Vlieger, Mar 24 2019 *)

PROG

(MAGMA) [n: n in [1..1000000] | IsIntegral((n - NumberOfDivisors(n)) * n / SumOfDivisors(n))]

(PARI) isok(m) = frac(m*(m-numdiv(m))/sigma(m)) == 0; \\ Michel Marcus, Mar 25 2019

CROSSREFS

Cf. A000005, A000203, A000396, A001599, A049820, A325021, A325022, A325023, A325024.

Sequence in context: A085286 A062085 A284632 * A147800 A246624 A212872

Adjacent sequences:  A325017 A325018 A325019 * A325021 A325022 A325023

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Mar 24 2019

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 29 05:39 EDT 2020. Contains 333105 sequences. (Running on oeis4.)