# dlangt (l) - Linux Man Pages

## NAME

DLANGT - returns the value of the one norm, or the Frobenius norm, or the infinity norm, or the element of largest absolute value of a real tridiagonal matrix A

## SYNOPSIS

DOUBLE PRECISION
FUNCTION DLANGT( NORM, N, DL, D, DU )

CHARACTER NORM

INTEGER N

DOUBLE PRECISION D( * ), DL( * ), DU( * )

## PURPOSE

DLANGT returns the value of the one norm, or the Frobenius norm, or the infinity norm, or the element of largest absolute value of a real tridiagonal matrix A.

## DESCRIPTION

DLANGT returns the value

DLANGT max(abs(A(i,j))), NORM aqMaq or aqmaq

(

norm1(A),         NORM aq1aq, aqOaq or aqoaq

(

normI(A),         NORM aqIaq or aqiaq

(

normF(A),         NORM aqFaq, aqfaq, aqEaq or aqeaq where norm1 denotes the one norm of a matrix (maximum column sum), normI denotes the infinity norm of a matrix (maximum row sum) and normF denotes the Frobenius norm of a matrix (square root of sum of squares). Note that max(abs(A(i,j))) is not a consistent matrix norm.

## ARGUMENTS

NORM (input) CHARACTER*1
Specifies the value to be returned in DLANGT as described above.
N (input) INTEGER
The order of the matrix A. N >= 0. When N = 0, DLANGT is set to zero.
DL (input) DOUBLE PRECISION array, dimension (N-1)
The (n-1) sub-diagonal elements of A.
D (input) DOUBLE PRECISION array, dimension (N)
The diagonal elements of A.
DU (input) DOUBLE PRECISION array, dimension (N-1)
The (n-1) super-diagonal elements of A.