A140773 Consider the products of all pairs of (not necessarily distinct) positive divisors of n. a(n) is the number of these products that divide n. a(n) also is the number of the products that are divisible by n.

1, 2, 2, 4, 2, 5, 2, 6, 4, 5, 2, 10, 2, 5, 5, 9, 2, 10, 2, 10, 5, 5, 2, 16, 4, 5, 6, 10, 2, 14, 2, 12, 5, 5, 5, 20, 2, 5, 5, 16, 2, 14, 2, 10, 10, 5, 2, 24, 4, 10, 5, 10, 2, 16, 5, 16, 5, 5, 2, 28, 2, 5, 10, 16, 5, 14, 2, 10, 5, 14, 2, 32, 2, 5, 10, 10, 5, 14, 2, 24, 9, 5, 2, 28, 5, 5, 5, 16, 2, 28, 5

Offset

1,2

Comments

Number of 3D grids of n congruent boxes with two different edge lengths, in a box, modulo rotation (cf. A034836 for cubes instead of boxes and A007425 for boxes with three different edge lengths; cf. A000005 for the 2D case). - Manfred Boergens, Feb 25 2021

Number of distinct faces obtainable by arranging n unit cubes into a cuboid. - Chris W. Milson, Mar 14 2021

Links

Antti Karttunen, Table of n, a(n) for n = 1..10000

Chris W. Milson, Constructing Cuboids

Chris W. Milson A faster algorithm for a(n)

Formula

a(n) = Sum_{m|n} A038548(m) = Sum_{m|n} ceiling(d(m)/2), where d(m) = number of divisors of m (A000005). - Manfred Boergens, Feb 25 2021

a(n) = Sum_{d|n} A135539(d,n/d). - Ridouane Oudra, Jul 10 2021

Example

The divisors of 20 are 1,2,4,5,10,20. There are 10 pairs of divisors whose product divides 20: 1*1=1, 1*2=2, 1*4=4, 1*5=5, 1*10=10, 1*20=20, 2*2=4, 2*5=10, 2*10=20, 4*5 = 20. Likewise, there are 10 products that are divisible by 20: 4*5=20, 2*10=20, 4*10=40, 10*10=100, 1*20=20, 2*20=40, 4*20=80, 5*20=100, 10*20=200, 20*20=400. So a(20) = 10.

Mathematica

(* first do *) Needs["Combinatorica`"] (* then *) f[n_] := Count[ n/Times @@@ Union[Sort /@ Tuples[Divisors@ n, 2]], _Integer]; Array[f, 91] (* Robert G. Wilson v, May 31 2008 *)
d=Divisors[n]; r=Length[d]; Sum[Ceiling[Length[Divisors[d[[j]]]]/2], {j, r}] (* Manfred Boergens, Feb 25 2021 *)

Prog

(PARI)
\\ Two implementations, after the two different interpretations given by the author of the sequence:
A140773v1(n) = { my(ds = divisors(n), s=0); for(i=1, #ds, for(j=i, #ds, if(!(n%(ds[i]*ds[j])), s=s+1))); s; }
A140773v2(n) = { my(ds = divisors(n), s=0); for(i=1, #ds, for(j=i, #ds, if(!((ds[i]*ds[j])%n), s=s+1))); s; }
\\ Antti Karttunen, May 19 2017
(Python) # See C. W. Milson link.

Crossrefs

Cf. A140774.

Cf. A000005, A034836, A038548.

Sequence in context: A054134 A319822 A005127 * A133911 A069932 A056148

Adjacent sequences: A140770 A140771 A140772 * A140774 A140775 A140776

Keyword

nonn

Author

Leroy Quet, May 29 2008

Extensions

Corrected and extended by Robert G. Wilson v, May 31 2008

Status

approved