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A030662 Number of combinations of n things from 1 to n at a time, with repeats allowed. 21
1, 5, 19, 69, 251, 923, 3431, 12869, 48619, 184755, 705431, 2704155, 10400599, 40116599, 155117519, 601080389, 2333606219, 9075135299, 35345263799, 137846528819, 538257874439, 2104098963719, 8233430727599, 32247603683099, 126410606437751, 495918532948103 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Add terms of an increasingly bigger diamond-shaped part of Pascal's triangle:

.......................... 1

............ 1 .......... 1 1

.. 1 ...... 1 1 ........ 1 2 1

. 1 1 =5 . 1 2 1 =19 .. 1 3 3 1 =69

.. 2 ...... 3 3 ........ 4 6 4

............ 6 ......... 10 10

.......................... 20

-  Ralf Stephan, May 17 2004

The prime p divides a((p-1)/2) for p = 5,13,17,29,37,41,53,61,73,89,97.. = A002144[n] Pythagorean primes: primes of form 4n+1. - Alexander Adamchuk, Jul 04 2006

Also, number of square submatrices of a square matrix. - Jono Henshaw (jjono(AT)hotmail.com), Apr 22 2008

Partial sums of A051924. - J. M. Bergot, Jun 22 2013

Number of partitions with Ferrers diagrams that fit in an n X n box (

excluding the empty partition of 0). - Michael Somos, Jun 02 2014

LINKS

T. D. Noe, Table of n, a(n) for n = 1..500

J. D. Horton and A. Kurn, Counting sequences with complete increasing subsequences, Congress Numerantium, 33 (1981), 75-80. MR 681905

M. Janjic and B. Petkovic, A Counting Function, arXiv preprint arXiv:1301.4550 [math.CO], 2013. - From N. J. A. Sloane, Feb 13 2013

Jianqiang Zhao, Uniform Approach to Double Shuffle and Duality Relations of Various q-Analogs of Multiple Zeta Values via Rota-Baxter Algebras, arXiv preprint arXiv:1412.8044 [math.NT], 2014.

FORMULA

a(n) = A000984(n) - 1.

a(n) = 2*(2*n-1)!/(n!*(n-1)!)-1.

a(n) = Sum_{k=1..n} binomial(n, k)^2. - Benoit Cloitre, Aug 20 2002

a(n) = Sum_{j=0..n} Sum_{i=j..n+j} binomial(i, j). - Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Jul 23 2003

a(n) = Sum_{i=0..n-1} Sum_{j=0..n-1} binomial(i+j, i). - N. J. A. Sloane, Jan 31 2009

Also for n>1: a(n)=(2*n)!/(n!)^2-1 - Hugo Pfoertner, Feb 10 2004

a(n) = Sum[Sum[(2n-i-j)!/(n-i)!/(n-j)!,{i,1,n}],{j,1,n}]. - Alexander Adamchuk, Jul 04 2006

a(n) = A115112(n) + 1. - Jono Henshaw (jjono(AT)hotmail.com), Apr 22 2008

G.f.: Q(0)*(1-4*x)/x - 1/x/(1-x), where Q(k)= 1 + 4*(2*k+1)*x/( 1 - 1/(1 + 2*(k+1)/Q(k+1))); (continued fraction). - Sergei N. Gladkovskii, May 11 2013

Conjecture: n*a(n) +2*(-3*n+2)*a(n-1) +(9*n-14)*a(n-2) +2*(-2*n+5)*a(n-3)=0. - R. J. Mathar, Jun 25 2013

0 = a(n)*(+16*a(n+1) - 70*a(n+2) + 68*a(n+3) - 14*a(n+4)) + a(n+1)*(-2*a(n+1) + 61*a(n+2) - 96*a(n+3) + 23*a(n+4)) + a(n+2)*(-6*a(n+2) + 31*a(n+3)  - 10*a(n+4)) + a(n+3)*(-2*a(n+3) + a(n+4)) for all n in Z. - Michael Somos, Jun 02 2014

From Ilya Gutkovskiy, Jan 25 2017: (Start)

O.g.f.: (1 - x - sqrt(1 - 4*x))/((1 - x)*sqrt(1 - 4*x)).

E.g.f.: exp(x)*(exp(x)*BesselI(0,2*x) - 1). (End)

EXAMPLE

G.f. = x + 5*x^2 + 19*x^3 + 69*x^4 + 251*x^5 + 923*x^6 + 3431*x^7 + ...

MAPLE

seq(sum((binomial(n, m))^2, m=1..n), n=1..23); # Zerinvary Lajos, Jun 19 2008

f:=n->add( add( binomial(i+j, i), i=0..n), j=0..n); [seq(f(n), n=0..12)]; # N. J. A. Sloane, Jan 31 2009

MATHEMATICA

Table[Sum[Sum[(2n-i-j)!/(n-i)!/(n-j)!, {i, 1, n}], {j, 1, n}], {n, 1, 20}] (* Alexander Adamchuk, Jul 04 2006 *)

a[n_] := 2*(2*n-1)!/(n*(n-1)!^2)-1; Table[a[n], {n, 1, 26}] (* Jean-Fran├žois Alcover, Oct 11 2012, from first formula *)

PROG

(Sage)

def a(n) : return binomial(2*n, n) - 1

[a(n) for n in (1..26)] # Peter Luschny, Apr 21 2012

(PARI) a(n)=binomial(2*n, n)-1 \\ Charles R Greathouse IV, Jun 26 2013

CROSSREFS

2*A001700 - 1.

Column k=2 of A047909.

Cf. A091908, A144660, A002144.

Central column of triangle A014473.

Right-hand column 2 of triangle A102541.

Sequence in context: A240525 A264200 A055991 * A149758 A026590 A243413

Adjacent sequences:  A030659 A030660 A030661 * A030663 A030664 A030665

KEYWORD

nonn,nice

AUTHOR

Donald Mintz (djmintz(AT)home.com)

STATUS

approved

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Last modified August 20 17:15 EDT 2017. Contains 290836 sequences.