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NAME SSTEBZ - compute the eigenvalues of a symmetric tridiagonal matrix T SYNOPSIS SUBROUTINE SSTEBZ( RANGE, ORDER, N, VL, VU, IL, IU, ABSTOL, D, E, M, NSPLIT, W, IBLOCK, ISPLIT, WORK, IWORK, INFO ) CHARACTER ORDER, RANGE INTEGER IL, INFO, IU, M, N, NSPLIT REAL ABSTOL, VL, VU INTEGER IBLOCK( * ), ISPLIT( * ), IWORK( * ) REAL D( * ), E( * ), W( * ), WORK( * ) PURPOSE SSTEBZ computes the eigenvalues of a symmetric tridiagonal matrix T. The user may ask for all eigenvalues, all eigen- values in the half-open interval (VL, VU], or the IL-th through IU-th eigenvalues. See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiago- nal Matrix", Report CS41, Computer Science Dept., Stanford University, July 21, 1966. ARGUMENTS RANGE (input) CHARACTER = 'A': ("All") all eigenvalues will be found. = 'V': ("Value") all eigenvalues in the half-open interval (VL, VU] will be found. = 'I': ("Index") the IL-th through IU-th eigenvalues (of the entire matrix) will be found. ORDER (input) CHARACTER = 'B': ("By Block") the eigenvalues will be grouped by split-off block (see IBLOCK, ISPLIT) and ordered from smallest to largest within the block. = 'E': ("Entire matrix") the eigenvalues for the entire matrix will be ordered from smallest to largest. N (input) INTEGER The dimension of the tridiagonal matrix T. N >= 0. VL (input) REAL If RANGE='V', the lower bound of the interval to be searched for eigenvalues. Eigenvalues less than or equal to VL will not be returned. Not referenced if RANGE='A' or 'I'. VU (input) REAL If RANGE='V', the upper bound of the interval to be searched for eigenvalues. Eigenvalues greater than VU will not be returned. VU must be greater than VL. Not referenced if RANGE='A' or 'I'. IL (input) INTEGER If RANGE='I', the index (from smallest to largest) of the smallest eigenvalue to be returned. IL must be at least 1. Not referenced if RANGE='A' or 'V'. IU (input) INTEGER If RANGE='I', the index (from smallest to largest) of the largest eigenvalue to be returned. IU must be at least IL and no greater than N. Not refer- enced if RANGE='A' or 'V'. ABSTOL (input) REAL The absolute tolerance for the eigenvalues. An eigenvalue (or cluster) is considered to be located if it has been determined to lie in an interval whose width is ABSTOL or less. If ABSTOL is less than or equal to zero, then ULP*|T| will be used, where |T| means the 1-norm of T. D (input) REAL array, dimension (N) The n diagonal elements of the tridiagonal matrix T. To avoid overflow, the matrix must be scaled so that its largest entry is no greater than overflow**(1/2) * underflow**(1/4) in absolute value, and for greatest accuracy, it should not be much smaller than that. E (input) REAL array, dimension (N-1) The (n-1) off-diagonal elements of the tridiagonal matrix T. To avoid overflow, the matrix must be scaled so that its largest entry is no greater than overflow**(1/2) * underflow**(1/4) in absolute value, and for greatest accuracy, it should not be much smaller than that. M (output) INTEGER The actual number of eigenvalues found. 0 <= M <= N. (See also the description of INFO=2,3.) NSPLIT (output) INTEGER The number of diagonal blocks in the matrix T. 1 <= NSPLIT <= N. W (output) REAL array, dimension (N) On exit, the first M elements of W will contain the eigenvalues. (SSTEBZ may use the remaining N-M ele- ments as workspace.) IBLOCK (output) INTEGER array, dimension (N) At each row/column j where E(j) is zero or small, the matrix T is considered to split into a block diagonal matrix. On exit, IBLOCK(i) specifies which block (from 1 to the number of blocks) the eigen- value W(i) belongs to. (SSTEBZ may use the remain- ing N-M elements as workspace.) ISPLIT (output) INTEGER array, dimension (N) The splitting points, at which T breaks up into sub- matrices. The first submatrix consists of rows/columns 1 to ISPLIT(1), the second of rows/columns ISPLIT(1)+1 through ISPLIT(2), etc., and the NSPLIT-th consists of rows/columns ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N. (Only the first NSPLIT elements will actually be used, but since the user cannot know a priori what value NSPLIT will have, N words must be reserved for ISPLIT.) WORK (workspace) REAL array, dimension (4*N) IWORK (workspace) INTEGER array, dimension (3*N) INFO (output) INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: some or all of the eigenvalues failed to con- verge or were not computed: =1 or 3: Bisection failed to converge for some eigenvalues; these eigenvalues are flagged by a negative block number. The effect is that the eigenvalues may not be as accurate as the absolute and relative tolerances. This is generally caused by unexpectedly inaccurate arithmetic. =2 or 3: RANGE='I' only: Not all of the eigenvalues IL:IU were found. Effect: M < IU+1-IL Cause: non-monotonic arithmetic, causing the Sturm sequence to be non-monotonic. Cure: recalculate, using RANGE='A', and pick out eigenvalues IL:IU. In some cases, increasing the PARAMETER "FUDGE" may make things work. = 4: RANGE='I', and the Gershgorin interval initially used was too small. No eigenvalues were computed. Probable cause: your machine has sloppy floating- point arithmetic. Cure: Increase the PARAMETER "FUDGE", recompile, and try again. PARAMETERS RELFAC REAL, default = 2.0e0 The relative tolerance. An interval (a,b] lies within "relative tolerance" if b-a < RELFAC*ulp*max(|a|,|b|), where "ulp" is the machine precision (distance from 1 to the next larger float- ing point number.) FUDGE REAL, default = 2 A "fudge factor" to widen the Gershgorin intervals. Ideally, a value of 1 should work, but on machines with sloppy arithmetic, this needs to be larger. The default for publicly released versions should be large enough to handle the worst machine around. Note that this has no effect on accuracy of the solution.