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# dlangt

```
NAME
DLANGT - return the value of the one norm, or the Frobenius
norm, or the infinity norm, or the element of largest abso-
lute 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 = 'M' or 'm'
(
( norm1(A),         NORM = '1', 'O' or 'o'
(
( normI(A),         NORM = 'I' or 'i'
(
( normF(A),         NORM = 'F', 'f', 'E' or 'e'

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  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.
```