LA_SYEVR / LA_HEEVR compute selected eigenvalues and,
optionally, the corresponding
eigenvectors of a real symmetric/complex Hermitian matrix .
Eigenvalues and eigenvectors can be selected by specifying
either a range of values or a range of indices for the desired
LA_SYEVR and LA_HEEVR use a relatively robust representation (RRR) algorithm. It is usually the fastest algorithm of all and uses the least workspace.