Root systems and optimal block designs

Peter Cameron
2009 The Michigan mathematical journal  
Motivated by a question of C.-S. Cheng on optimal block designs, this paper describes the symmetric matrices with entries 0, +1 and −1, zero diagonal, least eigenvalue strictly greater than −2, and constant row sum. I also describe briefly the motivation for the question. As a warmup, I consider the case where the least eigenvalue is −1.
doi:10.1307/mmj/1242071687 fatcat:cpddxqnzdnazratot6ukgqumom