Visible to Intel only — GUID: GUID-5B18E679-23D2-45CF-B15E-D97698E3130E
Visible to Intel only — GUID: GUID-5B18E679-23D2-45CF-B15E-D97698E3130E
?larre2a
Given a tridiagonal matrix, sets small off-diagonal elements to zero and for each unreduced block, finds base representations and eigenvalues.
Syntax
call slarre2a( range, n, vl, vu, il, iu, d, e, e2, rtol1, rtol2, spltol, nsplit, isplit, m, dol, dou, needil, neediu, w, werr, wgap, iblock, indexw, gers, sdiam, pivmin, work, iwork, minrgp, info )
call dlarre2a( range, n, vl, vu, il, iu, d, e, e2, rtol1, rtol2, spltol, nsplit, isplit, m, dol, dou, needil, neediu, w, werr, wgap, iblock, indexw, gers, sdiam, pivmin, work, iwork, minrgp, info )
Description
To find the desired eigenvalues of a given real symmetric tridiagonal matrix T, ?larre2a sets any "small" off-diagonal elements to zero, and for each unreduced block Ti, it finds
a suitable shift at one end of the block's spectrum,
the base representation, Ti - σiI = LiDiLiT, and
eigenvalues of each LiDiLiT.
The algorithm obtains a crude picture of all the wanted eigenvalues (as selected by range). However, to reduce work and improve scalability, only the eigenvalues dol to dou are refined. Furthermore, if the matrix splits into blocks, RRRs for blocks that do not contain eigenvalues from dol to dou are skipped. The DQDS algorithm (subroutine ?lasq2) is not used, unlike in the sequential case. Instead, eigenvalues are computed in parallel to some figures using bisection.
Product and Performance Information |
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Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex. Notice revision #20201201 |
Input Parameters
- range
-
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.
- n
-
INTEGER
The order of the matrix. n > 0.
- vl, vu
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
If range='V', the lower and upper bounds for the eigenvalues. Eigenvalues less than or equal to vl, or greater than vu, will not be returned. vl < vu.
If range='I' or ='A', ?larre2a computes bounds on the desired part of the spectrum.
- il, iu
-
INTEGER
If range='I', the indices (in ascending order) of the smallest and largest eigenvalues to be returned.
1 ≤il≤iu≤n.
- d
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size n
On entry, the n diagonal elements of the tridiagonal matrix T.
- e
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size n
The first (n-1) entries contain the subdiagonal elements of the tridiagonal matrix T; e(n) need not be set.
- e2
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size n
The first (n-1) entries contain the squares of the subdiagonal elements of the tridiagonal matrix T; e2(n) need not be set.
- rtol1, rtol2
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Parameters for bisection.
An interval [left,right] has converged if right - left < max( rtol1*gap, rtol2*max(|left|,|right|) )
- spltol
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
The threshold for splitting.
- dol, dou
-
INTEGER
If the user wants to work on only a selected part of the representation tree, he can specify an index range dol:dou.
Otherwise, the setting dol=1, dou=n should be applied.
Note that dol and dou refer to the order in which the eigenvalues are stored in w.
- work
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Workspace array of size 6*n
- iwork
-
INTEGER
Workspace array of size 5*n
- minrgp
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
The minimum relative gap threshold to decide whether an eigenvalue or a cluster boundary is reached.
OUTPUT Parameters
- vl, vu
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
If range='V', the lower and upper bounds for the eigenvalues. Eigenvalues less than or equal to vl, or greater than vu, are not returned. vl < vu.
If range='I' or range='A', ?larre2a computes bounds on the desired part of the spectrum.
- d
-
The n diagonal elements of the diagonal matrices Di.
- e
-
e contains the subdiagonal elements of the unit bidiagonal matrices Li. The entries e( isplit(i) ), 1 ≤i≤nsplit, contain the base points σi on output.
- e2
-
The entries e2( isplit( i ) ), 1 ≤i≤nsplit have been set to zero.
- nsplit
-
INTEGER
The number of blocks T splits into. 1 ≤nsplit≤n.
- isplit
-
INTEGER
Array of size n
The splitting points, at which T breaks up into blocks.
The first block consists of rows/columns 1 to isplit(1), the second of rows/columns isplit(1)+1 through isplit(2), etc., and the nsplit-th block consists of rows/columns isplit(nsplit-1)+1 through isplit(nsplit)=n.
- m
- INTEGER
The total number of eigenvalues (of all LiDiLiT) found.
- needil, neediu
- INTEGER
The indices of the leftmost and rightmost eigenvalues of the root node RRR which are needed to accurately compute the relevant part of the representation tree.
- w
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size n
The first m elements contain the eigenvalues. The eigenvalues of each of the blocks, LiDiLiT, are sorted in ascending order ( ?larre2a may use the remaining n-m elements as workspace).
Note that immediately after exiting this routine, only the eigenvalues from position dol:dou in w rely on this processor because the eigenvalue computation is done in parallel.
- werr
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size n
The error bound on the corresponding eigenvalue in w.
Note that immediately after exiting this routine, only the uncertainties from position dol:dou in werr are reliable on this processor because the eigenvalue computation is done in parallel.
- wgap
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size n
The separation from the right neighbor eigenvalue in w. The gap is only with respect to the eigenvalues of the same block as each block has its own representation tree.
Exception: at the right end of a block we store the left gap
Note that immediately after exiting this routine, only the gaps from position dol:dou in wgap are reliable on this processor because the eigenvalue computation is done in parallel.
- iblock
-
INTEGERArray of size n
The indices of the blocks (submatrices) associated with the corresponding eigenvalues in w; iblock(i)=1 if eigenvalue w(i) belongs to the first block from the top, iblock(i)=2 if w(i) belongs to the second block, and so on.
- indexw
-
INTEGERArray of size n
The indices of the eigenvalues within each block (submatrix); for example, indexw(i)= 10 and iblock(i)=2 imply that the i-th eigenvalue w(i) is the 10th eigenvalue in block 2.
- gers
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
Array of size 2*n
The n Gerschgorin intervals (the i-th Gerschgorin interval is (gers(2*i-1), gers(2*i))).
- pivmin
-
REAL for slarre2a
DOUBLE PRECISION for dlarre2a
The minimum pivot in the sturm sequence for T.
- info
-
INTEGER
= 0: successful exit
> 0: A problem occurred in ?larre2a.
< 0: One of the called subroutines signaled an internal problem. Needs inspection of the corresponding parameter info for further information.
=-1: Problem in ?larrd2.
=-2: Not enough internal iterations to find base representation.
=-3: Problem in ?larrb2 when computing the refined root representation.
=-4: Problem in ?larrb2 when preforming bisection on the desired part of the spectrum.
= -9 Problem: m < dou-dol+1, that is the code found fewer eigenvalues than it was supposed to.