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A337360 Sum of the coordinates of all pairs of divisors of n, (d1,d2), such that d1 <= d2. 2
2, 9, 12, 28, 18, 60, 24, 75, 52, 90, 36, 196, 42, 120, 120, 186, 54, 273, 60, 294, 160, 180, 72, 540, 124, 210, 200, 392, 90, 648, 96, 441, 240, 270, 240, 910, 114, 300, 280, 810, 126, 864, 132, 588, 546, 360, 144, 1364, 228, 651, 360, 686, 162, 1080, 360, 1080, 400, 450, 180, 2184 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

2*a(n) is the sum of the perimeters of all distinct rectangles that can be made whose side lengths are divisors of n.

Every divisor of n occurs tau(n) + 1 times in the coordinates of divisors of n. - David A. Corneth, Aug 25 2020

LINKS

David A. Corneth, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = Sum_{d1|n, d2|n, d1<=d2} (d1+d2).

a(n) = sigma(n) * (tau(n) + 1). - David A. Corneth, Aug 25 2020

EXAMPLE

a(3) = 12; The divisors of 3 are {1,3}. The divisor pairs, (d1,d2), where d1 <= d2 are (1,1), (1,3) and (3,3). The sum of all the coordinates is then 1+1+1+3+3+3 = 12. So a(3) = 12.

a(4) = 28; The divisors of 4 are {1,2,4}. The divisor pairs, (d1,d2), where d1 <= d2 are (1,1), (1,2), (1,4), (2,2), (2,4) and (4,4). The sum of all the coordinates is then 1+1+1+2+1+4+2+2+2+4+4+4 = 28. So a(4) = 28.

a(5) = 18; The divisors of 5 are {1,5}. The divisor pairs, (d1,d2), where d1 <= d2 are (1,1), (1,5) and (5,5). The sum of all the coordinates is then 1+1+1+5+5+5 = 18. So a(5) = 18.

a(6) = 60; The divisors of 6 are {1,2,3,6}. The divisor pairs, (d1,d2), where d1 <= d2 are (1,1), (1,2), (1,3), (1,6), (2,2), (2,3), (2,6), (3,3), (3,6), (6,6). The sum of all the coordinates is then 1+1+1+2+1+3+1+6+2+2+2+3+2+6+3+3+3+6+6+6 = 60. So a(6) = 60.

MATHEMATICA

Table[Sum[Sum[(i + k) (1 - Ceiling[n/k] + Floor[n/k]) (1 - Ceiling[n/i] + Floor[n/i]), {i, k}], {k, n}], {n, 80}]

PROG

(PARI) a(n) = sigma(n) * (numdiv(n)+1) \\ David A. Corneth, Aug 25 2020

CROSSREFS

Cf. A000005, A000203.

Sequence in context: A318681 A183207 A304797 * A340038 A178312 A253608

Adjacent sequences:  A337357 A337358 A337359 * A337361 A337362 A337363

KEYWORD

nonn,easy

AUTHOR

Wesley Ivan Hurt, Aug 24 2020

STATUS

approved

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Last modified July 23 11:34 EDT 2021. Contains 346259 sequences. (Running on oeis4.)