|
| |
|
|
A020487
|
|
Antiharmonic numbers: numbers n such that sigma_1(n) divides sigma_2(n).
|
|
13
| |
|
|
1, 4, 9, 16, 20, 25, 36, 49, 50, 64, 81, 100, 117, 121, 144, 169, 180, 196, 200, 225, 242, 256, 289, 324, 325, 361, 400, 441, 450, 468, 484, 500, 529, 576, 578, 605, 625, 650, 676, 729, 784, 800, 841, 900, 961, 968, 980, 1024, 1025, 1058, 1089, 1156, 1225, 1280, 1296
(list; graph; refs; listen; history; internal format)
|
|
|
|
OFFSET
| 1,2
|
|
|
COMMENTS
| Numbers k such that antiharmonic mean of divisors of k is an integer. Antiharmonic mean of divisors of number m = Product (p_i^e_i) is A001157(m)/A000203(m) = Product ((p_i^(e_i+1)+1)/(p_i+1)). a(n) = k = A001157(k)/A000203(k) for A001157(k)/A000203(k)is integer. (jaroslav.krizek(AT)atlas.cz) [From Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 09 2009]
|
|
|
EXAMPLE
| For n=3 the a(3)=9=3^2, antiharmonic mean of divisors of number 9 is (3^(2+1)+1)/(3+1)=7, 7 is integer. [From Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 09 2009]
|
|
|
CROSSREFS
| Cf. A001157, A000203. [From Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 09 2009]
Sequence in context: A066213 A010441 A046871 * A046870 A127702 A010461
Adjacent sequences: A020484 A020485 A020486 * A020488 A020489 A020490
|
|
|
KEYWORD
| nonn
|
|
|
AUTHOR
| David W. Wilson (davidwwilson(AT)comcast.net)
|
| |
|
|
