# dlacn2.f - Man Page

SRC/dlacn2.f

## Synopsis

### 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

N

```          N is INTEGER
The order of the matrix.  N >= 1.```

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).```

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.```

ISGN

`          ISGN is INTEGER array, dimension (N)`

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).```

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.```

ISAVE

```          ISAVE is INTEGER array, dimension (3)
ISAVE is used to save variables between calls to DLACN2```
Author

Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

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 135 of file dlacn2.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page dlacn2(3) is an alias of dlacn2.f(3).

Tue Nov 28 2023 12:08:42 Version 3.12.0 LAPACK