On the Exponent of Several Classes of Oscillatory Matrices

Oscillatory matrices were introduced in the seminal work of Gantmacher and Krein. An $n\times n$ matrix $A$ is called oscillatory if all its minors are nonnegative and there exists a positive integer $k$ such that all minors of $A^k$ are positive. The smallest $k$ for which this holds is called the exponent of the oscillatory matrix $A$. Gantmacher and Krein showed that the exponent is always smaller than or equal to $n-1$. An important and nontrivial problem is to determine the exact value of the exponent. Here we use the successive elementary bidiagonal factorization of oscillatory matrices, and its graph-theoretic representation, to derive an explicit expression for the exponent of several classes of oscillatory matrices, and a nontrivial upper-bound on the exponent for several other classes.