?larre2a

Developer Reference for Intel® oneAPI Math Kernel Library for C

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ID 766684

Date 12/16/2022

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?larre2a

Given a tridiagonal matrix, sets small off-diagonal elements to zero and for each unreduced block, finds base representations and eigenvalues.

Syntax

void slarre2a(char* range, MKL_INT* n, float* vl, float* vu, MKL_INT* il, MKL_INT* iu, float* d, float* e, float* e2, float* rtol1, float* rtol2, float* spltol, MKL_INT* nsplit, MKL_INT* isplit, MKL_INT* m, MKL_INT* dol, MKL_INT* dou, MKL_INT* needil, MKL_INT* neediu, float* w, float* werr, float* wgap, MKL_INT* iblock, MKL_INT* indexw, float* gers, float* sdiam, float* pivmin, float* work, MKL_INT* iwork, float* minrgp, MKL_INT* info);

void dlarre2a(char* range, MKL_INT* n, double* vl, double* vu, MKL_INT* il, MKL_INT* iu, double* d, double* e, double* e2, double* rtol1, double* rtol2, double* spltol, MKL_INT* nsplit, MKL_INT* isplit, MKL_INT* m, MKL_INT* dol, MKL_INT* dou, MKL_INT* needil, MKL_INT* neediu, double* w, double* werr, double* wgap, MKL_INT* iblock, MKL_INT* indexw, double* gers, double* sdiam, double* pivmin, double* work, MKL_INT* iwork, double* minrgp, MKL_INT* info);

Include Files

mkl_scalapack.h

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 T_i, it finds

a suitable shift at one end of the block's spectrum,
the base representation, T_i - σ_iI = L_iD_iL_i^T, and
eigenvalues of each L_iD_iL_i^T.

NOTE:

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 (function ?lasq2) is not used, unlike in the sequential case. Instead, eigenvalues are computed in parallel to some figures using bisection.

Product and Performance Information
Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex. Notice revision #20201201

Product and Performance Information

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.

Notice revision #20201201

Input Parameters

range

= 'A': ("All") all eigenvalues will be found.

= 'V': ("Value") all eigenvalues in the half-open interval (vl, vu] will be found.

= 'I': ("Index") eigenvalues of the entire matrix with the indices in a given range will be found.

n

The order of the matrix. n > 0.

vl, vu

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

If range='I', the indices (in ascending order) of the smallest eigenvalue, to be returned in w[il-1], and largest eigenvalue, to be returned in w[iu-1].

1 ≤il≤iu≤n.

d

Array of size n

On entry, the n diagonal elements of the tridiagonal matrix T.

e

Array of size n

The first (n-1) entries contain the subdiagonal elements of the tridiagonal matrix T; e[n-1] need not be set.

e2

Array of size n

The first (n-1) entries contain the squares of the subdiagonal elements of the tridiagonal matrix T; e2[n-1] need not be set.

rtol1, rtol2

Parameters for bisection.

An interval [left,right] has converged if right - left < max( rtol1*gap, rtol2*max(|left|,|right|) )

spltol

The threshold for splitting.

dol, dou

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

Workspace array of size 6*n

iwork

Workspace array of size 5*n

minrgp

The minimum relative gap threshold to decide whether an eigenvalue or a cluster boundary is reached.

OUTPUT Parameters

vl, vu

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 D_i.

e

e contains the subdiagonal elements of the unit bidiagonal matrices L_i. The entries e[isplit[i]], 0 ≤i<nsplit, contain the base points σ_i+1 on output.

e2

The entries e2[isplit[i ]], 0≤i<nsplit have been set to zero.

nsplit

The number of blocks T splits into. 1 ≤nsplit≤n.

isplit

Array of size n

The splitting points, at which T breaks up into blocks.

The first block consists of rows/columns 1 to isplit[0], the second of rows/columns isplit[0]+1 through isplit[1], etc., and the nsplit-th block consists of rows/columns isplit[nsplit-2]+1 through isplit[nsplit-1]=n.

m

The total number of eigenvalues (of all L_iD_iL_i^T) found.

needil, neediu

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

Array of size n

The first m elements contain the eigenvalues. The eigenvalues of each of the blocks, L_iD_iL_i^T, are sorted in ascending order ( ?larre2a may use the remaining n-m elements as workspace).

Note that immediately after exiting this function, only the eigenvalues in wwith indices in range dol-1:dou-1 rely on this processor because the eigenvalue computation is done in parallel.

werr

Array of size n

The error bound on the corresponding eigenvalue in w.

Note that immediately after exiting this function, only the uncertainties in werrwith indices in range dol-1:dou-1 are reliable on this processor because the eigenvalue computation is done in parallel.

wgap

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 function, only the gaps in wgapwith indices in range dol-1:dou-1 are reliable on this processor because the eigenvalue computation is done in parallel.

iblock

Array 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

Array 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+1)-th eigenvalue w[i] is the 10th eigenvalue in block 2.

gers

Array of size 2*n

The n Gerschgorin intervals (the i-th Gerschgorin interval is (gers[2*i-2], gers[2*i-1])).

pivmin

The minimum pivot in the sturm sequence for T.

info

= 0: successful exit

> 0: A problem occurred in ?larre2a.

< 0: One of the called functions 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.

Parent topic: ScaLAPACK Auxiliary Routines

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