SYNOPSIS
 SUBROUTINE DSPGVX(
 ITYPE, JOBZ, RANGE, UPLO, N, AP, BP, VL, VU, IL, IU, ABSTOL, M, W, Z, LDZ, WORK, IWORK, IFAIL, INFO )
 CHARACTER JOBZ, RANGE, UPLO
 INTEGER IL, INFO, ITYPE, IU, LDZ, M, N
 DOUBLE PRECISION ABSTOL, VL, VU
 INTEGER IFAIL( * ), IWORK( * )
 DOUBLE PRECISION AP( * ), BP( * ), W( * ), WORK( * ), Z( LDZ, * )
PURPOSE
DSPGVX computes selected eigenvalues, and optionally, eigenvectors of a real generalized symmetricdefinite eigenproblem, of the form A*x=(lambda)*B*x, A*Bx=(lambda)*x, or B*A*x=(lambda)*x. Here A and B are assumed to be symmetric, stored in packed storage, and B is also positive definite. Eigenvalues and eigenvectors can be selected by specifying either a range of values or a range of indices for the desired eigenvalues.ARGUMENTS
 ITYPE (input) INTEGER

Specifies the problem type to be solved:
= 1: A*x = (lambda)*B*x
= 2: A*B*x = (lambda)*x
= 3: B*A*x = (lambda)*x  JOBZ (input) CHARACTER*1

= 'N': Compute eigenvalues only;
= 'V': Compute eigenvalues and eigenvectors.  RANGE (input) CHARACTER*1

= 'A': all eigenvalues will be found.
= 'V': all eigenvalues in the halfopen interval (VL,VU] will be found. = 'I': the ILth through IUth eigenvalues will be found.  UPLO (input) CHARACTER*1

= 'U': Upper triangle of A and B are stored;
= 'L': Lower triangle of A and B are stored.  N (input) INTEGER
 The order of the matrix pencil (A,B). N >= 0.
 AP (input/output) DOUBLE PRECISION array, dimension (N*(N+1)/2)
 On entry, the upper or lower triangle of the symmetric matrix A, packed columnwise in a linear array. The jth column of A is stored in the array AP as follows: if UPLO = 'U', AP(i + (j1)*j/2) = A(i,j) for 1<=i<=j; if UPLO = 'L', AP(i + (j1)*(2*nj)/2) = A(i,j) for j<=i<=n. On exit, the contents of AP are destroyed.
 BP (input/output) DOUBLE PRECISION array, dimension (N*(N+1)/2)
 On entry, the upper or lower triangle of the symmetric matrix B, packed columnwise in a linear array. The jth column of B is stored in the array BP as follows: if UPLO = 'U', BP(i + (j1)*j/2) = B(i,j) for 1<=i<=j; if UPLO = 'L', BP(i + (j1)*(2*nj)/2) = B(i,j) for j<=i<=n. On exit, the triangular factor U or L from the Cholesky factorization B = U**T*U or B = L*L**T, in the same storage format as B.
 VL (input) DOUBLE PRECISION
 VU (input) DOUBLE PRECISION If RANGE='V', the lower and upper bounds of the interval to be searched for eigenvalues. VL < VU. Not referenced if RANGE = 'A' or 'I'.
 IL (input) INTEGER
 IU (input) INTEGER If RANGE='I', the indices (in ascending order) of the smallest and largest eigenvalues to be returned. 1 <= IL <= IU <= N, if N > 0; IL = 1 and IU = 0 if N = 0. Not referenced if RANGE = 'A' or 'V'.
 ABSTOL (input) DOUBLE PRECISION
 The absolute error tolerance for the eigenvalues. An approximate eigenvalue is accepted as converged when it is determined to lie in an interval [a,b] of width less than or equal to ABSTOL + EPS * max( a,b ) , where EPS is the machine precision. If ABSTOL is less than or equal to zero, then EPS*T will be used in its place, where T is the 1norm of the tridiagonal matrix obtained by reducing A to tridiagonal form. Eigenvalues will be computed most accurately when ABSTOL is set to twice the underflow threshold 2*DLAMCH('S'), not zero. If this routine returns with INFO>0, indicating that some eigenvectors did not converge, try setting ABSTOL to 2*DLAMCH('S').
 M (output) INTEGER
 The total number of eigenvalues found. 0 <= M <= N. If RANGE = 'A', M = N, and if RANGE = 'I', M = IUIL+1.
 W (output) DOUBLE PRECISION array, dimension (N)
 On normal exit, the first M elements contain the selected eigenvalues in ascending order.
 Z (output) DOUBLE PRECISION array, dimension (LDZ, max(1,M))
 If JOBZ = 'N', then Z is not referenced. If JOBZ = 'V', then if INFO = 0, the first M columns of Z contain the orthonormal eigenvectors of the matrix A corresponding to the selected eigenvalues, with the ith column of Z holding the eigenvector associated with W(i). The eigenvectors are normalized as follows: if ITYPE = 1 or 2, Z**T*B*Z = I; if ITYPE = 3, Z**T*inv(B)*Z = I. If an eigenvector fails to converge, then that column of Z contains the latest approximation to the eigenvector, and the index of the eigenvector is returned in IFAIL. Note: the user must ensure that at least max(1,M) columns are supplied in the array Z; if RANGE = 'V', the exact value of M is not known in advance and an upper bound must be used.
 LDZ (input) INTEGER
 The leading dimension of the array Z. LDZ >= 1, and if JOBZ = 'V', LDZ >= max(1,N).
 WORK (workspace) DOUBLE PRECISION array, dimension (8*N)
 IWORK (workspace) INTEGER array, dimension (5*N)
 IFAIL (output) INTEGER array, dimension (N)
 If JOBZ = 'V', then if INFO = 0, the first M elements of IFAIL are zero. If INFO > 0, then IFAIL contains the indices of the eigenvectors that failed to converge. If JOBZ = 'N', then IFAIL is not referenced.
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
> 0: DPPTRF or DSPEVX returned an error code:
<= N: if INFO = i, DSPEVX failed to converge; i eigenvectors failed to converge. Their indices are stored in array IFAIL. > N: if INFO = N + i, for 1 <= i <= N, then the leading minor of order i of B is not positive definite. The factorization of B could not be completed and no eigenvalues or eigenvectors were computed.
FURTHER DETAILS
Based on contributions byMark Fahey, Department of Mathematics, Univ. of Kentucky, USA