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A061718 a(n) = (n*(n+1)/2)^n. 4
1, 9, 216, 10000, 759375, 85766121, 13492928512, 2821109907456, 756680642578125, 253295162119140625, 103510234140112521216, 50714860157241037295616, 29345269354638035222576971 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) is the number of n X n matrices with nonnegative integer entries such that every row sum equals 2. - Sharon Sela (sharonsela(AT)hotmail.com), May 08 2002

Resultant of the polynomials P(n,x) and Q(n,x) where P(n,x)=sum(k=1,n,k*(-x)^k) and Q(n,x)=x^n-1. - Benoit Cloitre, Jan 26 2003

a(n) is also the number of positive-volume, axis-aligned, n-dimensional rectangular solids that have vertices in the set {0,1,...,n}^n. Proof: If (M_1,...,M_n) is the corner with the maximum coordinate values for such a solid, then there are (M_1)*...*(M_n) possibilities for the corner with the minimum coordinate values. The sum over all possibilities for M_1, ..., M_n can be factored into the product of n sums; each of the n sums simplifies to n(n+1)/2. - Lee A. Newberg, Aug 31 2009

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..100

FORMULA

Sum(i=1..n,j=1..n,k=1..n,...,(i*j*k*...)). E.g., a(2) = 9 because 1*1 + 1*2 + 2*1 + 2*2 = 9. - Ben Paul Thurston, Aug 15 2006

a(n) = [x^n] 1/(1 - (n*(n + 1)/2)*x). - Ilya Gutkovskiy, Oct 10 2017

MAPLE

a:=n->mul(sum(j, j=0..n), k=1..n): seq(a(n), n=1..13); # Zerinvary Lajos, Jun 02 2007

a:=n->mul(binomial(n+2, 2), k=0..n): seq(a(n), n=0..12); # Zerinvary Lajos, Oct 02 2007

PROG

(C++) /* e.g. n = 6: */

int main()

{

    int sum = 0;

    for(int i = 1; i < 7; i++)

        for(int j = 1; j < 7; j++)

            for(int k=1; k<7; k++)

                for(int l = 1; l < 7; l++)

                    for(int m = 1; m < 7; m++)

                        for(int n = 1; n < 7; n++)

                            sum += i*j*k*l*m*n;

    cout << sum << endl;

    return 0;

} // Ben Paul Thurston, Aug 15 2006

(PARI) { for (n=1, 100, write("b061718.txt", n, " ", (n*(n + 1)/2)^n) ) } \\ Harry J. Smith, Jul 26 2009

CROSSREFS

Cf. A066300.

Sequence in context: A217042 A064633 A084942 * A085741 A211044 A152288

Adjacent sequences:  A061715 A061716 A061717 * A061719 A061720 A061721

KEYWORD

easy,nonn

AUTHOR

Jason Earls, Jun 20 2001

STATUS

approved

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Last modified September 21 14:54 EDT 2020. Contains 337272 sequences. (Running on oeis4.)