

A002182


Highly composite numbers, definition (1): numbers n where d(n), the number of divisors of n (A000005), increases to a record.
(Formerly M1025 N0385)


302



1, 2, 4, 6, 12, 24, 36, 48, 60, 120, 180, 240, 360, 720, 840, 1260, 1680, 2520, 5040, 7560, 10080, 15120, 20160, 25200, 27720, 45360, 50400, 55440, 83160, 110880, 166320, 221760, 277200, 332640, 498960, 554400, 665280, 720720, 1081080, 1441440, 2162160
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OFFSET

1,2


COMMENTS

Where record values of d(n) occur: d(n) > d(k) for all k < n.
A002183 is the RECORDS transform of A000005, i.e., lists the corresponding values d(n) for n in A002182.
Flammenkamp's page also has a copy of the missing Siano paper.
Highly composite numbers are the product of primorials, A002110. See A112779 for the number of primorial terms in the product of a highly composite number.  Jud McCranie, Jun 12 2005
Sigma and tau for highly composite numbers through the 146th entry conform to a power fit as follows: log(sigma)=A*log(tau)^B where (A,B) =~ (1.45,1.38).  Bill McEachen, May 24 2006
a(n) often corresponds to P(n,m) = number of permutations of n things taken m at a time. Specifically, if start=1, pointers 16, 9, 10, 1315, 1719, 22, 23, 28, 34, 37, 43, 52, ... An example is a(37)=665280, which is P(12,6)=12!/(126)!.  Bill McEachen, Feb 09 2009
Concerning the previous comment, if m=1, then P(n,m) can represent any number. So let's assume m > 1. Searching the first 1000 terms, the only indices of terms of the form P(n,m) are 4, 5, 6, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 22, 23, 27, 28, 31, 34, 37, 41, 43, 44, 47, 50, 52, and 54. Note that a(44) = 4324320 = P(2079,2). See A163264.  T. D. Noe, Jun 10 2009
A large number of highly composite numbers have 9 as their digit root.  Parthasarathy Nambi, Jun 07 2009
Because 9 divides all highly composite numbers greater than 1680, those numbers have digital root 9.  T. D. Noe, Jul 24 2009
See A181309 for highly composite numbers that are not highly abundant.
a(n) is also defined by the recurrence: a(1) = 1, a(n+1)/sigma(a(n+1)) < a(n) / sigma(a(n)).  Michel Lagneau, Jan 02 2012 [NOTE: This "definition" is wrong (a(20)=7560 does not satisfy this inequality) and incomplete: It does not determine a sequence uniquely, e.g., any subsequence would satisfy the same relation. The intended meaning is probably the definition of the (different) sequence A004394.  M. F. Hasler, Sep 13 2012]
Up to a(1000), the terms beyond a(5) = 12 resp. beyond a(9) = 60 are a multiples of these. Is this true for all subsequent terms?  M. F. Hasler, Sep 13 2012 [Yes: see EXAMPLE in A199337!  M. F. Hasler, Jan 03 2020]
Differs from the superabundant numbers from a(20)=7560 on, which is not in A004394. The latter is not a subsequence of A002182, as might appear from considering the displayed terms: The two sequences have only 449 terms in common, the largest of which is A002182(2567) = A004394(1023). See A166735 for superabundant numbers that are not highly composite, and A004394 for further information.  M. F. Hasler, Sep 13 2012
Subset of A067128 and of A025487.  David A. Corneth, May 16 2016, Jan 03 2020
It seems that a(n) + 1 is often prime. For n <= 1000 there are 210 individual primes and 17 pairs of twin primes. See link to Lim's paper below.  Dmitry Kamenetsky, Mar 02 2019
There are infinitely many numbers in this sequence and a(n+1) <= 2*a(n), because it is sufficient to multiply a(n) by 2 to get a number having more divisors. (This proves Guess 0 in the Lim paper.) For n = (1, 2, 4, 5, 9, 13, 18, ...) one has equality in this bound, but asymptotically a(n+1)/a(n) goes to 1, cf. formula due to Erdős. See A068507 for the terms such that a(n)+1 are twin primes.  M. F. Hasler, Jun 23 2019
Conjecture: For n > 7, a(n) is a Zumkeller number (A083207). Verified for n up to and including 48. If this conjecture is true, one may base on it an alternative proof of the fact that for n>7 a(n) is not a perfect square (see Fact 5, Rao/Peng arXiv link at A083207).  Ivan N. Ianakiev, Jun 29 2019
The first instance of omega(a(n)) < omega(a(n1)) (omega = A001221: number of prime divisors) is at a(26) = 45360. Up to n = 10^4, 1759 terms have this property, but omega decreases by 2 only at indices n = 5857, 5914 and 5971.  M. F. Hasler, Jan 02 2020
Inequality (54) in Ramanujan (1915) implies that for any m there is n* such that m  a(n) for all n > n*: see A199337 for the proof.  M. F. Hasler, Jan 03 2020


REFERENCES

CRC Press Standard Mathematical Tables 28th Ed, p. 61.
J.M. De Koninck, Ces nombres qui nous fascinent, Entry 180, p. 56, Ellipses, Paris 2008.
L. E. Dickson, History of Theory of Numbers, I, p. 323.
Ellingsen Jr, Harold W. "A Fresh Look at Highly Composite Numbers." The American Mathematical Monthly 126.8 (2019): 740741.
R. Honsberger, An introduction to Ramanujan's Highly Composite Numbers, Chap. 14 pp. 193200 Mathematical Gems III, DME no. 9 MAA 1985
J. L. Nicolas, On highly composite numbers, pp. 215244 in Ramanujan Revisited, Editors G. E. Andrews et al., Academic Press 1988
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 128.


LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000 (Obtained from A. Flammenkamp's data; first 1000 terms from T. D. Noe)
Yu. Bilu, P. Habegger, L. Kühne, Effective bounds for singular units, arXiv:1805.07167 [math.NT], 2018.
Benjamin Braun, Brian Davis, Antichain Simplices, arXiv:1901.01417 [math.CO], 2019.
P. Erdős, On Highly composite numbers, J. London Math. Soc. 19 (1944), 130133 MR7,145d; Zentralblatt 61,79.
A. Flammenkamp, Highly composite numbers
A. Flammenkamp, List of the first 1200 highly composite numbers
A. Flammenkamp, List of the first 779,674 highly composite numbers
James Grime and Brady Haran, 5040 and other AntiPrime Numbers, Numberphile video (2016)
B. Hinman, Letter to N. J. A. Sloane, Aug. 1980
Aneesh M. Koya, P. P. Deepthi, Plug and play selfconfigurable IoT gateway node for telemonitoring of ECG, Computers in Biology and Medicine (2019) Vol. 112, 103359.
J. C. Lagarias, An elementary problem equivalent to the Riemann hypothesis, Am. Math. Monthly 109 (#6, 2002), 534543; arXiv:math/0008177 [math.NT], 20002001.
W. Lauritzen, Versatile Numbers Versatile Economics
Benny Lim, Prime Numbers Generated From Highly Composite Numbers, Parabola Magazine, volume 54, issue 3, 2018.
R. J. Mathar, Maple program to convert the Flammenkamp file to an OEIS bfile.
R. J. Mathar, Output of above Maple program. [Uncompresses to 9.1 MB]
Graeme McRae, Highly Composite Numbers.
J.L. Nicolas, Ordre maximal d'un element du groupe S_n de permutations et 'highly composite numbers' (Text in French).
J.L. Nicolas and G. Robin, Highly Composite Numbers by Srinivasa Ramanujan, The Ramanujan Journal, Vol. 1(2), pp. 119153, Kluwer Academics Pub.
K. O'Bryant, PlanetMath.org, Highly composite number.
S. Ramanujan, Highly composite numbers, Proceedings of the London Mathematical Society ser. 2, vol. XIV, no.1 (1915): 347409. (DOI: 10.1112/plms/s2_14.1.347, also available with an additional footnote in the PDF at http://ramanujan.sirinudi.org/Volumes/published/ram15.html)
S. Ratering, An interesting subset of the highly composite numbers, Math. Mag., 64 (1991), 343346.
G. Robin, Méthodes d'optimisation pour un problème de théorie des nombres, RAIRO Informatique Théorique, 17, 1983, 239247.
Vladimir Shevelev, On Erdős constant, arXiv:1605.08884 [math.NT], 2016.
D. B. Siano and J. D. Siano, An Algorithm for Generating Highly Composite Numbers.
N. J. A. Sloane, Transforms.
M. Waldschmidt, From highly composite numbers to transcendental number theory, 2013.
Eric Weisstein's World of Mathematics, Highly Composite Number.
Wikipedia, Highly composite number.


FORMULA

Also, for n >= 2, smallest values of p for which A006218(p)A006318(p1) = A002183(n).  Philippe LALLOUET (philip.lallouet(AT)wanadoo.fr), Jun 23 2007
a(n+1) < a(n) * (1+log(a(n))^c) for some positive c (see Erdős).  David A. Corneth, May 16 2016
a(n) = A108951(A329902(n)).  Antti Karttunen, Jan 08 2020


EXAMPLE

a(5) = 12 is in the sequence because A000005(12) is larger than any earlier value in A000005.  M. F. Hasler, Jan 03 2020


MATHEMATICA

a = 0; Do[b = DivisorSigma[0, n]; If[b > a, a = b; Print[n]], {n, 1, 10^7}]
(* Convert A. Flammenkamp's 779674term dataset; first, decompress, rename "HCN.txt": *)
a = Times @@ {Times @@ Prime@ Range@ ToExpression@ First@ #1, If[# == {}, 1, Times @@ MapIndexed[Prime[First@ #2]^#1 &, #]] &@ DeleteCases[1 + Flatten@ Map[If[StringFreeQ[#, "^"], ToExpression@ #, ConstantArray[#1, #2] & @@ ToExpression@ StringSplit[#, "^"]] &, #2], 0]} & @@ TakeDrop[StringSplit@ #, 1] & /@ Import["HCN.txt", "Data"] (* Michael De Vlieger, May 08 2018 *)


PROG

(PARI) print1(r=1); forstep(n=2, 1e5, 2, if(numdiv(n)>r, r=numdiv(n); print1(", "n))) \\ Charles R Greathouse IV, Jun 10 2011
(PARI) v002182 = Vec(1, 1000) /* Vector for memoization. */; A002182(n) = { if(n <= #v002182 && v002182[n], v002182[n], my(i=min(n, #v002182+1), k, d, s=1); if(n >= i, v002182 = Vec(v002182, n*9\8)); until(v002182[i=1], ); k = v002182[i]; while( i<n, d = numdiv(k); if(s<60 && k>=60, s=60); until(numdiv(k+=s) > d, ); v002182[i+=1] = k); k)} \\ [Could be made more efficient using even larger steps.]  Antti Karttunen, Jun 06 2017; edited by M. F. Hasler, Jan 03 2020
(Python)
from sympy import divisor_count
A002182_list, r = [], 0
for i in range(1, 10**4):
d = divisor_count(i)
if d > r:
r = d
A002182_list.append(i) # Chai Wah Wu, Mar 23 2015


CROSSREFS

Cf. A000005 (number of divisors), A002110, A002183, A002473, A004394, A025487, A106037, A108602, A112778, A112779, A112780, A112781, A006218, A126098, A002201, A072938, A094348, A003418, A161184, A037992 (infinitary analog), A108951, A329902.
Cf. A261100 (a left inverse).
Cf. A002808.  Peter J. Marko, Aug 16 2018
Cf. A279930 (highly composite and highly Brazilian).
Cf. A068507 (terms such that a(n)+1 are twin primes).
Cf. A199337 (number of terms not divisible by n).
Sequence in context: A181804 A094348 A242298 * A077006 A166981 A004394
Adjacent sequences: A002179 A002180 A002181 * A002183 A002184 A002185


KEYWORD

nonn,nice,changed


AUTHOR

N. J. A. Sloane


EXTENSIONS

Jun 19 1996: Changed beginning to start at 1.
Jul 10 1996: Matthew Conroy points out that these are different from the superabundant numbers  see A004394. Last 8 terms sent by J. Lowell; checked by Jud McCranie.
Description corrected by Gerard Schildberger and N. J. A. Sloane, Apr 04 2001
Additional references from Lekraj Beedassy, Jul 24 2001


STATUS

approved



