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A066446 Number of unordered divisor pairs of n. 11
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

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..65537 (terms 1..1000 from Harry J. Smith)

FORMULA

a(p) = 1 iff p is a prime.

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 a(n) = C(d(n), 2) = d(n)*{ d(n)-1 }/2 which is a triangular number (A000217).

a(n) = A184389(n) - A000005(n) = A035116(n) - A184389(n). - Reinhard Zumkeller, Sep 08 2015

a(n) = A000217(A000005(n)-1). - Antti Karttunen, Sep 21 2018

a(n) = Sum_{k|n, i|n, i < k} 1. - Wesley Ivan Hurt, Aug 20 2020

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) ) } \\ Harry J. Smith, Feb 15 2010

(Haskell)

a066446 = a000217 . subtract 1 . a000005'

-- Reinhard Zumkeller, Sep 08 2015

CROSSREFS

Cf. A000005, A000217, A129510.

Cf. A035116, A184389.

Sequence in context: A019570 A239303 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

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Last modified April 11 05:30 EDT 2021. Contains 342886 sequences. (Running on oeis4.)