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

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A232215 Number of n X n matrices (up to permutation of their rows and columns) with nonnegative integer entries with all row and column sums equal to 3. 3
 1, 1, 2, 5, 12, 31, 103, 383, 1731, 9273, 57563, 406465, 3212131, 28009976, 266688867, 2749264797, 30480560319, 361435864747, 4562860845767, 61084137737436, 864206301930764, 12882343725953858, 201788397502682460, 3313420771907580764, 56910480298885139055 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Arises from counting of symmetric tensor invariants without color. See Geloun-Ramgoolam, Section 6.2 for information and Mathematica code. LINKS Mehmet Emin Aktas, Dessins d'Enfants of Trigonal Curves, arXiv:1706.09956 [math.AG], 2017, Theorem 5. J. B. Geloun, S. Ramgoolam, Counting Tensor Model Observables and Branched Covers of the 2-Sphere, arXiv preprint arXiv:1307.6490 [hep-th], 2013. Brendan McKay, Number of all different n-by-n matrices where sum of rows and columns is 3, MathOverflow, 2016. B. D. McKay and N. C. Wormald, Autormorphisms of Random Graphs with Specified Vertices. Combinatorica 4 (4) (1984) 325-338. FORMULA a(n) = 1 + Sum_{i=1..n} A328159(i). - Brendan McKay, Oct 05 2019 EXAMPLE a(2) = 2 because there are 2 such 2 X 2 matrices: [1 2;2 1] and [3 0;0 3]. - Nathaniel Johnston, Oct 12 2016 CROSSREFS Column k=3 of A333733. Cf. A328159. Sequence in context: A090826 A132441 A000840 * A265265 A293868 A162434 Adjacent sequences:  A232212 A232213 A232214 * A232216 A232217 A232218 KEYWORD nonn AUTHOR N. J. A. Sloane, Nov 22 2013 EXTENSIONS New name and a(9)-a(11) from Nathaniel Johnston, Oct 12 2016 a(12) and a(13) from Brendan McKay, Oct 05 2019 a(0)=1 prepended, a(12)-a(13) corrected and terms a(14) and beyond from Andrew Howroyd, Apr 04 2020 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 December 1 16:44 EST 2021. Contains 349430 sequences. (Running on oeis4.)