A001121 Number of doubly-regular tournaments of order 4n-1.

1, 1, 2, 2, 37, 722

Offset

2,3

Comments

Equally, number of skew digraphs in switching classes of Hadamard matrices of order 4n.

From Elijah Beregovsky, Feb 17 2026: (Start)

A regular tournament T is called doubly-regular if for every vertex pair {u, v} in T, the number of common out-neighbors of u and v is the same constant.

These are also normally regular digraphs NRD(4n-1,2n-1,n-1,.).

a(8) >= 13330 (see Al-Azemi). (End)

Links

Table of n, a(n) for n=2..7.

Abdullah Al-Azemi, Classification of regular digraphs, normally regular digraphs, and strongly regular digraphs, IJPAM, Volume 78, No. 3 (2012).

J. Gebel, Integer points on Mordell curves [Cached copy, after the original web site tnt.math.se.tmu.ac.jp was shut down in 2017]

Leif K. Jørgensen, Normally Regular Digraphs, arXiv:1410.8424 [math.CO], 2014.

Kenneth Brooks Reid and Ezra Brown, Doubly regular tournaments are equivalent to skew hadamard matrices, J. Comb. Th., Series A, Vol. 12, Issue 3, May 1972, 332-338.

Edward Spence, Classification of Hadamard matrices of order 24 and 28, Discrete Math. 140 (1995), no. 1-3, 185-243.

Crossrefs

Sequence in context: A267345 A059523 A038623 * A394299 A202711 A222668

Adjacent sequences: A001118 A001119 A001120 * A001122 A001123 A001124

Keyword

nonn,hard,more,nice,changed

Author

N. J. A. Sloane

Extensions

Better name suggested by Brendan McKay, Oct 31 2007

Status

approved