  The expanded Aztec diamond is a generalized version of the Aztec diamond,
with an arbitrary number of long columns and long rows in the middle. In this
paper, we count the number of domino tilings of the expanded Aztec diamond. The
exact number of domino tilings is given by recurrence relations of state
matrices by virtue of the state matrix recursion algorithm, recently developed
by the author to solve various two-dimensional regular lattice model
enumeration problems.
