The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A230880 Number of 2-packed matrices with exactly n nonzero entries. 3
 1, 2, 8, 80, 1120, 20544, 463744, 12422656, 384947200, 13541822464, 533049493504, 23210958688256, 1107652218822656, 57482801016422400, 3223015475535380480, 194157345516262588416, 12505948470244176953344, 857670052436844788318208, 62395270194815987194789888 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS A k-packed matrix of size n X n is a matrix with entries in the alphabet A_k = {0,1, ..., k} such that each row and each column contains at least one nonzero entry. LINKS Andrew Howroyd, Table of n, a(n) for n = 0..100 H. Cheballah, S. Giraudo, R. Maurice, Combinatorial Hopf algebra structure on packed square matrices, arXiv preprint arXiv:1306.6605 [math.CO], 2013. FORMULA Cheballah et al. give an explicit formula. From Andrew Howroyd, Sep 20 2017: (Start) a(n) = Sum_{r=1..n} Sum_{i=0..r} Sum_{j=0..r} (-1)^(i+j) * binomial(r,i) * binomial(r,j) * binomial(i*j,n) * 2^n. a(n) = 2^n * A104602(n). (End) MATHEMATICA b[n_] := Sum[StirlingS1[n, k]*Sum[(m!)^2*StirlingS2[k, m]^2, {m, 0, k}], {k, 0, n}]/n!; a[n_] := 2^n*b[n]; Table[a[n], {n, 0, 18}] (* Jean-François Alcover, Oct 08 2017, translated from PARI *) PROG (PARI) \\ here b(n) is A104602. b(n) = {sum(m=0, n, sum(k=0, n, stirling(n, k, 1) * m!^2 * stirling(k, m, 2)^2)) / n!} a(n) = 2^n * b(n); \\ Andrew Howroyd, Sep 20 2017 CROSSREFS Cf. A048291, A055599, A230878, A230879, A104602. Sequence in context: A259705 A073561 A258970 * A214689 A202999 A308088 Adjacent sequences:  A230877 A230878 A230879 * A230881 A230882 A230883 KEYWORD nonn AUTHOR N. J. A. Sloane, Nov 09 2013 EXTENSIONS Terms a(9) and beyond from Andrew Howroyd, Sep 20 2017 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 22 22:19 EDT 2021. Contains 343197 sequences. (Running on oeis4.)