|
| |
|
|
A066446
|
|
Number of unordered divisor pairs of n.
|
|
5
|
|
|
|
0, 1, 1, 3, 1, 6, 1, 6, 3, 6, 1, 15, 1, 6, 6, 10, 1, 15, 1, 15, 6, 6, 1, 28, 3, 6, 6, 15, 1, 28, 1, 15, 6, 6, 6, 36, 1, 6, 6, 28, 1, 28, 1, 15, 15, 6, 1, 45, 3, 15, 6, 15, 1, 28, 6, 28, 6, 6, 1, 66, 1, 6, 15, 21, 6, 28, 1, 15, 6, 28, 1, 66, 1, 6, 15, 15, 6, 28, 1, 45, 10, 6, 1, 66, 6, 6, 6, 28
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
|
OFFSET
|
1,4
|
|
|
COMMENTS
|
a(n) = 1 iff n is a prime.
|
|
|
LINKS
|
Harry J. Smith, Table of n, a(n) for n=1,...,1000
|
|
|
FORMULA
|
Combinations of d(n), the number of divisors of n (A000005), taken two at a time. If the canonical factorization of n into prime powers is Product p^e(p) then d(n) = Product (e(p) + 1). Therefore C( d(n), 2) = d(n)*{ d(n)-1 }/2 which is a triangular number (A000217).
|
|
|
EXAMPLE
|
The divisors of 6 are 1, 2, 3 & 6. In unordered pairs they are {1, 2}, {1, 3}, {1, 6}, {2, 3}, {2, 6}, & {3, 6}. Since there are six pairs, a(6) = 6. Also d(6) = 4. 4*3/2 = 6.
|
|
|
MATHEMATICA
|
Table[ Binomial[ DivisorSigma[0, n], 2], {n, 1, 100}]
|
|
|
PROG
|
(PARI) { for (n=1, 1000, a=binomial(numdiv(n), 2); write("b066446.txt", n, " ", a) ) } [From Harry J. Smith, Feb 15 2010]
|
|
|
CROSSREFS
|
Cf. A000005, A000217, A129510.
Sequence in context: A068436 A019570 A040011 * A069625 A111614 A193279
Adjacent sequences: A066443 A066444 A066445 * A066447 A066448 A066449
|
|
|
KEYWORD
|
easy,nonn
|
|
|
AUTHOR
|
Robert G. Wilson v, Dec 28 2001
|
|
|
STATUS
|
approved
|
| |
|
|
