## Putnam 2005 A4

Let $H$ be an $n \times n$ matrix all of whose entries are $\pm 1$ and whose rows are mutually orthogonal. Suppose $H$ has an $a \times b$ sub matrix whose entries are all $1$. Show that $ab \leq n$.

Source: Putnam 2005

0

0

0

0

0

0

0

0

0

0