With the help of the computer, we have shown that in any coloring with two colors of the triangles on a set of 13 points there must exist a monochromatic tetrahedron. This proves the new upper bound R (4,4;3) < = 13. The previous best upper bound of 15 was derived independently by Giraud (1969 ), Schwenk (1978 ) and Sidorenko (1980 ). The first construction of a R (4,4;3)-good hypergraph on 12 points was presented by Isbell (1969 ), and the same one again more elegantly by Sidorenko (1980 ). We have constructed more than 200,000 R (4,4;3)-good hypergraphs on 12 points, but probably not the full set. R (4,4;3)=13 is the first known exact value of a classical Ramsey number for hypergraphs.
The solution was achieved with the help of a variety of algorithms relying on a strong connection between the colorings with two colors of the triangles on n points and the so-called Tura´n set systems T(n ,5,4). The main criterion used to prune the search space for R (4,4;3)-good hypergraphs was to count the number of 4-sets containing two triangles of each color; such families of 4-sets are known to form Tura´n systems and their cardinalities must be minorized by the corresponding Tura´n numbers T(n ,5,4). We used an innovative method for generating large families of set systems which efficiently prevents isomorphic copies of set systems being produced. This method has many potential applications to other general computer searches for elusive combinatorial configurations. As a check on the correctness of the algorithms, many of the intermediate subfamilies of R (4,4;3)-good hypergraphs were generated by two different methods: from colorings of triangles on a smaller number of points and independently via Tura´n systems. An important component of the software used was a general set-system automorphism group program .
Date of creation, presentation, or exhibit
Department, Program, or Center
Computer Science (GCCIS)
Brendan D. McKay and Stanislaw P. Radziszowski. 1991. The first classical Ramsey number for hypergraphs is computed. In Proceedings of the second annual ACM-SIAM symposium on Discrete algorithms (SODA '91). Society for Industrial and Applied Mathematics, Philadelphia, PA, USA, 304-308.
RIT – Main Campus