Purpose

LA_SYEVR / LA_HEEVR compute selected eigenvalues and, optionally, the corresponding eigenvectors of a real symmetric/complex Hermitian matrix $A$. Eigenvalues and eigenvectors can be selected by specifying either a range of values or a range of indices for the desired eigenvalues.
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.