?lacn2

Developer Reference for Intel® oneAPI Math Kernel Library for C

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ID 766684

Date 10/15/2026

Version

Public

?lacn2

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

Syntax

lapack_int LAPACKE_slacn2(lapack_int n, float *v, float *x, lapack_int *isgn, float *est,
                          lapack_int *kase, lapack_int *isave);

lapack_int LAPACKE_clacn2(lapack_int n, lapack_complex_float *v, lapack_complex_float *x,
                          float *est, lapack_int *kase, lapack_int *isave);

lapack_int LAPACKE_dlacn2(lapack_int n, double *v, double *x, lapack_int *isgn,
                          double *est, lapack_int *kase, lapack_int *isave);

lapack_int LAPACKE_zlacn2(lapack_int n, lapack_complex_double *v,
                          lapack_complex_double *x, double *est, lapack_int *kase,
                          lapack_int *isave);

Include Files

mkl.h

Description

The routine estimates the 1-norm of a square, real or complex matrix A . Reverse communication is used for evaluating matrix-vector products.

Input Parameters

n: Specifies the order of the matrix A . The value of n must be at least zero.

v , x: REAL for slacn2 DOUBLE PRECISION for dlacn2 COMPLEX for clacn2 DOUBLE COMPLEX for zlacn2 . Arrays, size ( n ) each. v is a workspace array. x is used as input after an intermediate return.
isgn: INTEGER . Workspace array, size ( n ), used with real flavors only.
est: REAL for slacn2 / clacn2 DOUBLE PRECISION for dlacn2 / zlacn2 On entry with kase set to 1 or 2, and isave(1) = 1 , est must be unchanged from the previous call to the routine.
kase: INTEGER . On the initial call to the routine, kase must be set to 0.

isave

Array, size ( 3 ).

Contains variables from the previous call to the routine.

Output Parameters

est: An estimate (a lower bound) for norm( A ).
kase: On an intermediate return, kase is set to 1 or 2, indicating whether x is overwritten by A * x or A^T * x for real flavors and A * x or A^H * x for complex flavors. On the final return, kase is set to 0.
v: On the final return, v = A*w , where est = norm(v)/norm(w) ( w is not returned).
x: On an intermediate return, x is overwritten by A*x , if kase = 1 , A^{T}*x , if kase = 2 (for real flavors), A^{H}*x , if kase = 2 (for complex flavors), and the routine must be re-called with all the other parameters unchanged.
isave: This parameter is used to save variables between calls to the routine.

Return Values

This function returns a value info .

If info = 0 , the execution is successful.
If info = -i , the i -th parameter had an illegal value.

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Developer Reference for Intel® oneAPI Math Kernel Library for C

?lacn2

Syntax

Include Files

Description

Input Parameters

Output Parameters

Return Values