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A027423 Number of positive divisors of n!. 67
1, 1, 2, 4, 8, 16, 30, 60, 96, 160, 270, 540, 792, 1584, 2592, 4032, 5376, 10752, 14688, 29376, 41040, 60800, 96000, 192000, 242880, 340032, 532224, 677376, 917280, 1834560, 2332800, 4665600, 5529600, 7864320, 12165120, 16422912 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

It appears that a(n+1)=2*a(n) if n is in A068499. - Benoit Cloitre, Sep 07 2002

Because a(0) = 1 and for all n > 0, 2*a(n) >= a(n+1), the sequence is a complete sequence. - Frank M Jackson, Aug 09 2013

Luca and Yound prove that a(n) divides n! for n >= 6. - Michel Marcus, Nov 02 2017

LINKS

T. D. Noe, Table of n, a(n) for n = 0..1000

D. Berend et al., Gaps between consecutive divisors of factorials, Ann. Inst. Fourier, 43 (3) (1993), 569-583.

Paul Erdős, S. W. Graham, Alexsandr Ivić, and Carl Pomerance, On the number of divisors of n!, Analytic Number Theory, Proceedings of a Conference in Honor of Heini Halberstam, ed. by B. C. Berndt, H. G. Diamond, A. J. Hildebrand, Birkhauser 1996, pp. 337-355.

Florian Luca and Paul Thomas Young, On the number of divisors of n! and of the Fibonacci numbers, Glasnik Matematicki, Vol. 47, No. 2 (2012), 285-293. DOI: 10.3336/gm.47.2.05.

Wikipedia, Complete sequence

Index entries for sequences related to factorial numbers

Index entries for sequences related to divisors of numbers

FORMULA

a(n) <= a(n+1) <= 2*a(n) - Benoit Cloitre, Sep 07 2002

From Avik Roy (avik_3.1416(AT)yahoo.co.in), Jan 28 2009: (Start)

Assume, p1,p2...pm are the prime numbers less than or equal to n.

bk=sum_{i=1,2,3...} floor(n/pk^i)

Then, a(n)= product_{i=1,2...,m} (bi+1)

For example, if n=5, p1=2,p2=3,p3=5

b1=floor(5/2)+floor(5/2^2)+floor(5/2^3)+...=2+1+0+..=3 similarly, b2=b3=1

Thus a(5)=(3+1)(1+1)(1+1)=16 (End)

a(n) = A000005(A000142(n)). - Michel Marcus, Sep 13 2014

EXAMPLE

a(4) = 8 because 4!=24 has precisely eight distinct divisors: 1, 2, 3, 4, 6, 8, 12, 24.

MAPLE

A027423 := n -> numtheory[tau](n!);

MATHEMATICA

Table[ DivisorSigma[0, n! ], {n, 0, 35}]

PROG

(PARI) for(k=0, 50, print1(numdiv(k!), ", ")) \\ Jaume Oliver Lafont, Mar 09 2009

(PARI) a(n)=my(s=1, t, tt); forprime(p=2, n, t=tt=n\p; while(tt, t+=tt\=p); s*=t+1); s \\ Charles R Greathouse IV, Feb 08 2013

(Haskell)

a027423 n = f 1 $ map (\p -> iterate (* p) p) a000040_list where

   f y ((pps@(p:_)):ppss)

     | p <= n = f (y * (sum (map (div n) $ takeWhile (<= n) pps) + 1)) ppss

     | otherwise = y

-- Reinhard Zumkeller, Feb 27 2013

CROSSREFS

Cf. A000005, A000142, A062569, A161466 (divisors of 10!).

Sequence in context: A164259 A164203 A164178 * A140410 A213368 A216212

Adjacent sequences:  A027420 A027421 A027422 * A027424 A027425 A027426

KEYWORD

nonn,easy,nice

AUTHOR

Glen Burch (gburch(AT)erols.com), Leroy Quet.

STATUS

approved

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Last modified October 16 12:07 EDT 2019. Contains 328056 sequences. (Running on oeis4.)