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A032741 a(0) = 0; for n > 0, a(n) = number of proper divisors of n (divisors of n which are less than n). 108
0, 0, 1, 1, 2, 1, 3, 1, 3, 2, 3, 1, 5, 1, 3, 3, 4, 1, 5, 1, 5, 3, 3, 1, 7, 2, 3, 3, 5, 1, 7, 1, 5, 3, 3, 3, 8, 1, 3, 3, 7, 1, 7, 1, 5, 5, 3, 1, 9, 2, 5, 3, 5, 1, 7, 3, 7, 3, 3, 1, 11, 1, 3, 5, 6, 3, 7, 1, 5, 3, 7, 1, 11, 1, 3, 5, 5, 3, 7, 1, 9, 4, 3, 1, 11, 3, 3, 3, 7, 1, 11, 3, 5, 3, 3, 3, 11, 1, 5, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

Number of d < n which divide n.

Call an integer k between 1 and n a "semi-divisor" of n if n leaves a remainder of 1 when divided by k, i.e., n == 1 (mod k). a(n) gives the number of semi-divisors of n+1. - Joseph L. Pe, Sep 11 2002

a(n+1) is also the number of k, 0<=k<=n-1, such that C(n,k) divides C(n,k+1). - Benoit Cloitre, Oct 17 2002

a(n+1) is also the number of factors of the n-th degree polynomial x^n + x^(n-1) + x^(n-2) + ... + x^2 + x + 1. Example: 1+x+x^2+x^3 = (1+x)(1+x^2) implies a(4)=2.

a(n) is also the number of factors of the n-th Fibonacci polynomial. - T. D. Noe, Mar 09 2006

Number of partitions of n into 2 parts with the second dividing the first. - Franklin T. Adams-Watters, Sep 20 2006

Number of partitions of n+1 into exactly one q and at least one q+1. Example: a(12)=5; indeed, we have 13 = 7 + 6 = 5 + 4 + 4 = 4 + 3 + 3 + 3 = 3 + 2 + 2 + 2 + 2 + 2 = 2 + 11*1.

Differences of A002541. - George Beck, Feb 12 2012

For n > 1: number of ones in row n+1 of triangle A051778. - Reinhard Zumkeller, Dec 03 2014

For n > 0, a(n) is the number of strong divisors of n. - Omar E. Pol, May 03 2015

REFERENCES

André Weil, Number Theory, An approach through history, From Hammurapi to Legendre, Birkhäuser, 1984, page 5.

LINKS

T. D. Noe, Table of n, a(n) for n = 0..10000

Eric Weisstein's World of Mathematics, Proper divisors

FORMULA

a(n) = tau(n)-1 = A000005(n)-1. Cf. A039653.

G.f.: Sum_(n>=1, x^(2*n)/(1-x^n) ). - Michael Somos, Apr 29, 2003

G.f.: Sum_(i>=1, (1-x^i+x^(2*i))/(1-x^i)). - Jon Perry, Jul 03 2004

a(n) = Sum_(k=1..n/2, A051731(n-k,k) ). - Reinhard Zumkeller, Nov 01 2009

G.f.: 2*Sum_{n>=1} Sum_{d|n} log(1 - x^(n/d))^(2*d) / (2*d)!. - Paul D. Hanna, Aug 21 2014

Dirichlet g.f.: zeta(s)*(zeta(s)-1). - Geoffrey Critzer, Dec 06 2014

a(n) = Sum_{k=1..n-1} binomial((n-1) mod k, k-1). - Wesley Ivan Hurt, Sep 26 2016

a(n) = Sum_{i=1..n-1} floor(n/i)-floor((n-1)/i). - Wesley Ivan Hurt, Nov 15 2017

EXAMPLE

a(6) = 3 since the proper divisors of 6 are 1, 2, 3.

MAPLE

A032741 := proc(n)

    if n = 0 then

        0 ;

    else

        numtheory[tau](n)-1 ;

    end if;

end proc: # R. J. Mathar, Feb 03 2013

MATHEMATICA

Prepend[DivisorSigma[0, Range[99]]-1, 0] (* Jayanta Basu, May 25 2013 *)

PROG

(PARI) a(n) = if(n<1, 0, numdiv(n)-1)

(PARI) {a(n)=polcoeff(2*sum(m=1, n\2+1, sumdiv(m, d, log(1-x^(m/d) +x*O(x^n) )^(2*d)/(2*d)!)), n)} \\ Paul D. Hanna, Aug 21 2014

(Haskell)

a032741 n = if n == 0 then 0 else a000005 n - 1

-- Reinhard Zumkeller, Jul 31 2014

CROSSREFS

Column 2 of A122934.

Cf. A003238, A001065, A027749, A027751 (list of proper divisors).

Cf. A051778, A070824.

Sequence in context: A199570 A239707 A294928 * A046051 A025812 A263001

Adjacent sequences:  A032738 A032739 A032740 * A032742 A032743 A032744

KEYWORD

nonn,easy,changed

AUTHOR

Patrick De Geest, May 15 1998

EXTENSIONS

Typos in definition corrected by Omar E. Pol, Dec 13 2008

STATUS

approved

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Last modified November 24 07:58 EST 2017. Contains 295173 sequences.