  Cyclic reduction is a method for the solution of (block-)tridiagonal linear
systems. In this note we review the method tailored to hermitian positive
definite banded linear systems.
  The reviewed method has the following advantages: It is numerically stable
without pivoting. It is suitable for parallel computations. In the presented
form, it uses fewer computations by exploiting symmetry. Like Cholesky, the
reviewed method breaks down when the matrix is not positive definite, offering
a robust way for determining positive definiteness.
