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A063123 Number of solutions (r,s), 0< r< s, to the equation 1/n = 1/r + 1/s + 1/(r*s). 2
1, 2, 3, 3, 4, 4, 4, 6, 6, 4, 6, 6, 4, 8, 10, 5, 6, 6, 6, 12, 8, 4, 8, 12, 6, 8, 12, 6, 8, 8, 6, 12, 8, 8, 18, 9, 4, 8, 16, 8, 8, 8, 6, 18, 12, 4, 10, 15, 9, 12, 12, 6, 8, 16, 16, 16, 8, 4, 12, 12, 4, 12, 21, 14, 16, 8, 6, 12, 16, 8, 12, 12, 4, 12, 18, 12, 16, 8, 10, 25, 10, 4, 12, 24, 8, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Unordered solutions to the equation 1/n = 1/r+1/s+1/(r*s) are r=d+n, s=n*(n+1)/d+n, where d is factor of n*(n+1) not greater than n.

Number of divisors of n-th oblong number not greater than n. - Chandler

LINKS

Ray Chandler, Table of n, a(n) for n=1..10000

FORMULA

a(n) = tau(n)*tau(n+1)/2 = A092517(n)/2.

EXAMPLE

a(2)=2 because 1/2=1/3+1/8+1/24=1/4+1/5+1/20.

PROG

(PARI) a(n) = numdiv(n)*numdiv(n+1)/2 \\ Michel Marcus, Jun 17 2013

CROSSREFS

Cf. A063520.

Sequence in context: A316388 A029107 A209727 * A086333 A240792 A217121

Adjacent sequences:  A063120 A063121 A063122 * A063124 A063125 A063126

KEYWORD

nonn

AUTHOR

Vladeta Jovovic, Aug 08 2001

STATUS

approved

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Last modified November 20 12:37 EST 2018. Contains 317402 sequences. (Running on oeis4.)