## Saturday, February 23, 2008

### Matrices with big determinant

The Hadamard inequality roughly says that the determinant of an $n \times n$ matrix whose entries are real numbers with absolute value at most $1$ is not greater than $n^{n/2}$. Can you find arbitrarily large matrices with entries of absolute value at most $1$ for which the equality holds (ie the determinant is exactly $n^{n/2}$)? You don't have to find such an $n \times n$ matrix for each $n$ (for some $n$'s this may be impossible, while for others this is a very difficult, unsolved problem), just find a family for arbitrarily large $n$. Can you also give a quick proof of the inequality? Note: the inequality also holds for complex-valued matrices, but you don't have to use complex numbers to solve this problem.

Bonus problem: is it possible to have equality if the matrix is a real $3 \times 3$ matrix?

Update (09/03/2008): Solution posted! (show solution)