|
Search: id:A090240
|
|
|
| A090240 |
|
Numbers n which occur as the harmonic mean of the divisors of m for some m. |
|
+0
9
|
|
| 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 17, 19, 21, 24, 25, 26, 27, 29, 31, 35, 37, 39, 41, 42, 44, 45, 46, 47, 48, 49, 50, 51, 53, 54, 60, 61, 70, 73, 75, 77, 78, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 91, 92, 94, 96, 97, 99, 101, 102, 105, 106, 107, 108, 110, 114, 115
(list; graph; listen)
|
|
|
OFFSET
|
1,2
|
|
|
COMMENT
|
The equation n = m*tau(m)/sigma(m) has an integer solution m.
Here tau(n) (A000005) is the number of divisors of n and sigma(n) is the sum of the divisors of n (A000203).
A001600 sorted in order.
|
|
REFERENCES
|
T. Goto and S. Shibata, All numbers whose positive divisors have integral harmonic mean up to 300, Math. Comput. 73 (2004), 475-491.
For further references see A001599.
|
|
MATHEMATICA
|
f[n_] := (n*DivisorSigma[0, n]/DivisorSigma[1, n]); a = Table[ 0, {120}]; Do[ b = f[n]; If[ IntegerQ[b] && b < 121 && a[[b]] == 0, a[[b]] = n], {n, 1, 560000000}]; Select[ Range[120], a[[ # ]] > 0 &] (from Robert G. Wilson v)
|
|
CROSSREFS
|
m's are in A091911.
Cf. A001599, A001600, A090758, A090759, A090760, A090761, A090762.
Sequence in context: A039213 A119605 A144146 this_sequence A137407 A167209 A074137
Adjacent sequences: A090237 A090238 A090239 this_sequence A090241 A090242 A090243
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
R. K. Guy (rkg(AT)cpsc.ucalgary.ca), Feb 08 2004
|
|
EXTENSIONS
|
More terms from Robert G. Wilson v, Feb 14 2004
|
|
|
Search completed in 0.002 seconds
|
