# slaneg.f - Man Page

## Synopsis

### Functions/Subroutines

integer function **slaneg** (**N**, D, LLD, SIGMA, PIVMIN, R)**SLANEG** computes the Sturm count.

## Function/Subroutine Documentation

### integer function slaneg (integer N, real, dimension( * ) D, real, dimension( * ) LLD, real SIGMA, real PIVMIN, integer R)

**SLANEG** computes the Sturm count.

**Purpose:**

SLANEG computes the Sturm count, the number of negative pivots encountered while factoring tridiagonal T - sigma I = L D L^T. This implementation works directly on the factors without forming the tridiagonal matrix T. The Sturm count is also the number of eigenvalues of T less than sigma. This routine is called from SLARRB. The current routine does not use the PIVMIN parameter but rather requires IEEE-754 propagation of Infinities and NaNs. This routine also has no input range restrictions but does require default exception handling such that x/0 produces Inf when x is non-zero, and Inf/Inf produces NaN. For more information, see: Marques, Riedy, and Voemel, "Benefits of IEEE-754 Features in Modern Symmetric Tridiagonal Eigensolvers," SIAM Journal on Scientific Computing, v28, n5, 2006. DOI 10.1137/050641624 (Tech report version in LAWN 172 with the same title.)

**Parameters:***N*N is INTEGER The order of the matrix.

*D*D is REAL array, dimension (N) The N diagonal elements of the diagonal matrix D.

*LLD*LLD is REAL array, dimension (N-1) The (N-1) elements L(i)*L(i)*D(i).

*SIGMA*SIGMA is REAL Shift amount in T - sigma I = L D L^T.

*PIVMIN*PIVMIN is REAL The minimum pivot in the Sturm sequence. May be used when zero pivots are encountered on non-IEEE-754 architectures.

*R*R is INTEGER The twist index for the twisted factorization that is used for the negcount.

**Author:**Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**December 2016

**Contributors:**Osni Marques, LBNL/NERSC, USA

Christof Voemel, University of California, Berkeley, USA

Jason Riedy, University of California, Berkeley, USA

Definition at line 120 of file slaneg.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page slaneg(3) is an alias of slaneg.f(3).

Tue Nov 14 2017 Version 3.8.0 LAPACK