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 A027423 Number of divisors of n!. 116
 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 Young prove that a(n) divides n! for n >= 6. - Michel Marcus, Nov 02 2017 LINKS Seiichi Manyama, Table of n, a(n) for n = 0..10000 (terms 0..1000 from T. D. Noe) Daniel Berend and J. E. Harmse, 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. Then, a(n) = Product_{i=1..m} (bi+1), where bk = Sum_{i=1..m} floor(n/pk^i). 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 a(n) ~ exp(c * n/log(n) + O(n/log(n)^2)), where c = A131688 (Erdős et al., 1996). - Amiram Eldar, Nov 07 2020 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 (Python 3.8+) from math import prod from collections import Counter from sympy import factorint def A027423(n): return prod(e+1 for e in sum((Counter(factorint(i)) for i in range(2, n+1)), start=Counter()).values()) # Chai Wah Wu, Jun 25 2022 CROSSREFS Cf. A000005, A000142, A062569, A131688, A161466 (divisors of 10!). Sequence in context: A164203 A164178 A335542 * 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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