The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”). Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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. 5
 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 (list; graph; refs; listen; history; text; internal format)
 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 9 08:41 EST 2021. Contains 349627 sequences. (Running on oeis4.)