The OEIS is supported by the many generous donors to the OEIS Foundation.

A325024
Multiply-perfect numbers m from A007691 such that m*(m-tau(m))/sigma(m) is not an integer where k-tau(k) is the number of the non-divisors of k (A049820) and sigma(k) is the sum of the divisors of k (A000203).
6
120, 523776, 459818240, 1476304896, 31998395520, 51001180160, 518666803200, 30823866178560, 740344994887680, 796928461056000, 212517062615531520, 69357059049509038080, 87934476737668055040, 170206605192656148480, 1161492388333469337600, 1802582780370364661760
OFFSET
1,1
Numbers m such that m divides sigma(m) but sigma(m) does not divide m*(m-tau(m)).
Complement of A325023 with respect to A007691.
EXAMPLE
120 is a term because 120*(120-tau(120))/sigma(120) = 120*(120-16)/360 = 104/3.
MATHEMATICA
Select[Range[10^6], And[Mod[#3, #1] == 0, !IntegerQ[#1 (#1 - #2)/#3]] & @@ Prepend[DivisorSigma[{0, 1}, #], #] &] (* Amiram Eldar, Jul 10 2019 after Michael De Vlieger at A325023 *)
PROG
(Magma) [n: n in [1..1000000] | not IsIntegral(((n-NumberOfDivisors(n)) * n) / SumOfDivisors(n)) and IsIntegral(SumOfDivisors(n)/n)]
(PARI) isA325024(m) = { my(s=sigma(m)); ((1==denominator(s/m)) && (1!=denominator(m*(m-numdiv(m))/s))); }; \\ Antti Karttunen, May 25 2019
CROSSREFS
Sequence in context: A009767 A181750 A003789 * A325026 A046987 A127232
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, May 12 2019
STATUS
approved

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

Last modified September 20 13:04 EDT 2024. Contains 376072 sequences. (Running on oeis4.)