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A226898 Hooley's Delta function: maximum number of divisors of n in [u, eu] for all u. (Here e is Euler's number 2.718... = A001113.) 5
1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 3, 1, 2, 2, 2, 1, 2, 1, 3, 2, 2, 1, 4, 1, 2, 1, 2, 1, 3, 1, 2, 1, 2, 2, 3, 1, 2, 1, 4, 1, 3, 1, 2, 2, 2, 1, 4, 1, 2, 1, 2, 1, 2, 2, 3, 1, 2, 1, 4, 1, 2, 2, 2, 2, 2, 1, 2, 1, 3, 1, 4, 1, 2, 2, 2, 2, 2, 1, 4, 1, 2, 1, 4, 1, 2, 1, 2, 1, 4, 2, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This functions measures the tendency of divisors of a number to cluster.

Tenenbaum (1985) proves that a(1) + ... + a(n) < n exp(c sqrt(log log n log log log n)) for some constant c > 0 and all n > 16. In particular, the average order of a(n) is O((log n)^k) for any k > 0.

Maier & Tenenbaum show that (log log n)^(g + o(1)) < a(n) < (log log n)^(log 2 + o(1)) for almost all n, with g = log 2/log((1-1/log 27)/(1-1/log 3)) = 0.338....

For generalizations, see de la Bretèche & Tenenbaum, Brüdern, Hall & Tenenbaum, and Caballero.

REFERENCES

J. M. R. Caballero, "Symmetric Dyck Paths and Hooley’s ∆-function", Combinatorics on Words, Springer International Publishing AG (2017).

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

R. de la Bretèche and G. Tenenbaum, Oscillations localisées sur les diviseurs, J. Lond. Math. Soc. 2 85:3 (2012), pp. 669-693.

Jörg Brüdern, Daniel's twists of Hooley's Delta function, Contributions in Analytic and Algebraic Number Theory, Springer Proceedings in Mathematics 9 (2012), pp 31-82.

Paul Erdős, On abundant-like numbers, Canad. Math. Bull. 17 (1974), pp. 599-602.

Paul Erdős and Jean-Louis Nicolas, Méthodes probabilistes et combinatoires en théorie des nombres, Bulletin des Sciences Mathématiques 2 (1976), pp. 301-320.

P. Erdős and J.-L. Nicolas, Répartition des nombres superabondants, Bull. Soc. Math. France 103 (1975), pp. 65-90.

R. R. Hall and G. Tenenbaum, The average orders of Hooley's Δ_r-functions, Mathematika 31:1 (1984), pp. 98-109.

R. R. Hall and G. Tenenbaum, The average orders of Hooley's Δ_r-functions, II, Compositio Math. 60 (1986), pp. 163-186.

C. Hooley, On a new technique and its applications to the theory of numbers, Proc. London Math. Soc. 3 38:1 (1979), pp. 115-151.

Helmut Maier and Gérald Tenenbaum, On the set of divisors of an integer, Invent. Math. 76 (1984), pp. 121-128.

Helmut Maier and Gérald Tenenbaum, On the normal concentration of divisors,  J. London Math. Soc. 2 31:3 (1985), pp. 393-400.

Helmut Maier and Gérald Tenenbaum, On the normal concentration of divisors. II., Math. Proc. Cambridge Philos. Soc. 147:3 (2009), pp. 513-540.

J.-L. Nicolas, Méthodes probabilistes et combinatoires en théorie des nombres, Séminaire Delange-Pisot-Poitou. Théorie des nombres, Tome 17 (1975-1976) no. 1, Exposé no. 9,  p. 1

J. M. Rodríguez Caballero, Symmetric Dyck Paths and Hooley's Δ-Function, In: Brlek S., Dolce F., Reutenauer C., Vandomme É. (eds) Combinatorics on Words, WORDS 2017, Lecture Notes in Computer Science, vol 10432.

Gérald Tenenbaum, Sur la concentration moyenne des diviseurs, Commentarii Mathematici Helvetici 60:1 (1985), pp. 411-428.

Index entries for "core" sequences

FORMULA

a(mn) <= d(m)a(n) where d(n) is A000005.

EXAMPLE

The divisors of 24 are 1, 2, 3, 4, 6, 8, 12, 24. For u = 3, {3, 4, 6, 8} are in [3, 3e] = [3, 8.15...] and thus a(24) = 4.

MAPLE

with(numtheory):

a:= n-> (l-> max(seq(nops(select(x-> is(x<=exp(1)*l[i]), l))-i+1,

        i=1..nops(l))))(sort([divisors(n)[]])):

seq(a(n), n=1..100);  # Alois P. Heinz, Jun 21 2013

MATHEMATICA

a[n_] := Module[{d = Divisors[n], m = 1}, For[i = 1, i < Length[d], i++, t = E*d[[i]]; m = Max[ Sum[ Boole[d[[j]] < t], {j, i, Length[d]}], m]]; m]; Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Oct 08 2013, after Pari *)

PROG

(PARI) a(n)=my(d=divisors(n), m=1); for(i=1, #d-1, my(t=exp(1)*d[i]); m=max(sum(j=i, #d, d[j]<t), m)); m

(PARI) a(n)=my(d=divisors(n), r, t); for(i=1, #d\2, t=setsearch(d, d[i]*exp(1)\1, 1); t=if(t, t-i, setsearch(d, d[i]*exp(1)\1)+1-i); if(t>r, r=t)); r \\ Charles R Greathouse IV, Mar 01 2018

(Haskell)

a226898 = maximum . map length .

   map (\ds@(d:_) -> takeWhile (<= e' d) ds) . init . tails . a027750_row

   where e' = floor . (* e) . fromIntegral; e = exp 1

-- Reinhard Zumkeller, Jul 06 2013

(Python)

from sympy import divisors, exp

def a(n):

    d=divisors(n)

    m=1

    for i in xrange(len(d) - 1):

        t=exp(1)*d[i]

        m=max(sum([1 if d[j]<t else 0 for j in xrange(i, len(d))]), m)

    return m

print map(a, xrange(1, 151)) # Indranil Ghosh, Jul 19 2017

CROSSREFS

Partial sums are A226901. Cf. A226899, A226900, A027750, A022843.

Sequence in context: A160981 A058656 A184221 * A033111 A078313 A025827

Adjacent sequences:  A226895 A226896 A226897 * A226899 A226900 A226901

KEYWORD

nonn,nice,core

AUTHOR

Charles R Greathouse IV, Jun 21 2013

STATUS

approved

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Last modified October 15 13:06 EDT 2019. Contains 328030 sequences. (Running on oeis4.)