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Design theory is the subfield of combinatorics concerning the existence and construction of highly symmetric arrangements. Finite projective planes, latin squares, and Steiner triple systems are examples of designs.
1
vote
Minimum covers of complete graphs by $4$-cycles
If n is even, you can form a partial edge cover with disjoint four-cycles: pick points u and v, and cover all edges coming from u and from v (except for uv) by disjoint cycles. Now recurse, leaving n/ …
6
votes
Status of Hadamard matrix conjecture
With all due respect to Colin McLarty, here is (in my not so humble opinion) a better answer.
The conjecture (For every positive integer $k,$ there is a square matrix $H$ of order $4k$ such that $H$ …
3
votes
Maximum determinant of $\{0,1\}$-valued $n\times n$-matrices
( I assume the R in the title is the real numbers. For arbitrary rings R an order needs
to be specified before remarks like those below should be made. )
Using the method suggested by Robin Chapman, …