LAPACK  3.4.2 LAPACK: Linear Algebra PACKage
dlacn2.f File Reference

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Functions/Subroutines

subroutine dlacn2 (N, V, X, ISGN, EST, KASE, ISAVE)
DLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vector products.

Function/Subroutine Documentation

 subroutine dlacn2 ( integer N, double precision, dimension( * ) V, double precision, dimension( * ) X, integer, dimension( * ) ISGN, double precision EST, integer KASE, integer, dimension( 3 ) ISAVE )

DLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vector products.

Purpose:
``` DLACN2 estimates the 1-norm of a square, real matrix A.
Reverse communication is used for evaluating matrix-vector products.```
Parameters:
 [in] N ``` N is INTEGER The order of the matrix. N >= 1.``` [out] V ``` V is DOUBLE PRECISION array, dimension (N) On the final return, V = A*W, where EST = norm(V)/norm(W) (W is not returned).``` [in,out] X ``` X is DOUBLE PRECISION array, dimension (N) On an intermediate return, X should be overwritten by A * X, if KASE=1, A**T * X, if KASE=2, and DLACN2 must be re-called with all the other parameters unchanged.``` [out] ISGN ` ISGN is INTEGER array, dimension (N)` [in,out] EST ``` EST is DOUBLE PRECISION On entry with KASE = 1 or 2 and ISAVE(1) = 3, EST should be unchanged from the previous call to DLACN2. On exit, EST is an estimate (a lower bound) for norm(A). ``` [in,out] KASE ``` KASE is INTEGER On the initial call to DLACN2, KASE should be 0. On an intermediate return, KASE will be 1 or 2, indicating whether X should be overwritten by A * X or A**T * X. On the final return from DLACN2, KASE will again be 0.``` [in,out] ISAVE ``` ISAVE is INTEGER array, dimension (3) ISAVE is used to save variables between calls to DLACN2```
Date:
September 2012
Further Details:
```  Originally named SONEST, dated March 16, 1988.

This is a thread safe version of DLACON, which uses the array ISAVE
in place of a SAVE statement, as follows:

DLACON     DLACN2
JUMP     ISAVE(1)
J        ISAVE(2)
ITER     ISAVE(3)```
Contributors:
Nick Higham, University of Manchester
References:
N.J. Higham, "FORTRAN codes for estimating the one-norm of a real or complex matrix, with applications to condition estimation", ACM Trans. Math. Soft., vol. 14, no. 4, pp. 381-396, December 1988.

Definition at line 137 of file dlacn2.f.

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