%I #28 Aug 22 2020 04:28:27
%S 1,1,1,1,0,1,1,4,0
%N Number of nonisomorphic projective planes of order n.
%C The Bruck-Ryser theorem says that a(n)=0 if n == 1 or 2 (mod 4) and is not the sum of two squares.
%D CRC Handbook of Combinatorial Designs, 1996, p. 695.
%D Handbook of Combinatorics, North-Holland '95, p. 672.
%H C. W. H. Lam, <a href="http://www.cecm.sfu.ca/organics/papers/lam/index.html">Publications</a>
%H C. W. H. Lam, <a href="http://www.maa.org/sites/default/files/pdf/upload_library/22/Ford/Lam305-318.pdf">The Search for a Finite Projective Plane of Order 10</a>, American Mathematical Monthly, 98, (no. 4) 1991, 305 - 318.
%H C. W. H. Lam, G. Kolesova and S. Swiercz, <a href="http://dx.doi.org/10.1016/0012-365X(91)90280-F">A computer search for finite projective planes of order 9</a>, Discrete Math., 92 (1991), 187-195.
%H C. W. H. Lam, L. Thiel and S. Swiercz, <a href="https://doi.org/10.4153/CJM-1989-049-4">The non-existence of finite projective planes of order 10</a>, Canad. J. Math., 41 (1989), 1117-1123.
%H G. Eric Moorhouse, <a href="https://ericmoorhouse.org/pub/planes/">Projective Planes of Small Order</a>
%H N. J. A. Sloane, <a href="http://neilsloane.com/doc/sg.txt">My favorite integer sequences</a>, in Sequences and their Applications (Proceedings of SETA '98).
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ProjectivePlane.html">Projective Planes</a>
%K nonn,hard,more,nice
%O 2,8
%A _N. J. A. Sloane_.