Compute the eigenvalues and eigenvectors of a real symmetric tridiagonal matrix.
Usage
call EigValVecSymTri (ain, n, eval, evec, ul, exitstatus)
Parameters
ain: input, real(dp), dimension (n,n)- The input real symmetric tridiagonal matrix.
n: input, integer(int32)- The rank of the matrix
ain. eval: output, real(dp), dimension (n)- The eigenvalues of
ain, sorted from largest to smallest. evec: output, real(dp), dimension (n,n)- The eigenvectors of
ain, sorted from largest to smallest eigenvalues. The sign of the first element of each eigenvector is chosen to be positive. ul: optional, input, character, default =L- If
Uthen the upper portion of the matrixainwill be used. IfLthen the lower portion of the matrixainwill be used (default). exitstatus: output, optional, integer(int32)- If present, instead of executing a STOP when an error is encountered, the variable exitstatus will be returned describing the error. 0 = No errors; 1 = Improper dimensions of input array; 2 = Improper bounds for input variable; 3 = Error allocating memory; 4 = File IO error.
Description
EigValVecSymTri will calculate the eigenvalues and eigenvectors of a real symmetric tridiagonal matrix. By default, only the lower portion of the matrix is used, but this can be changed by the optional argument ul. The eigenvalues and eigenvectors are sorted from largest to smallest eigenvalues, and the sign of the first element of each eigenvector is chosen to be positive. This routine factors the matrix ain using the LAPACK routine DSTEGR.