# ?langt

Returns the value of the 1-norm, Frobenius norm, infinity-norm, or the largest absolute value of any element of a general tridiagonal matrix.

## Syntax

val = slangt( norm, n, dl, d, du )

val = dlangt( norm, n, dl, d, du )

val = clangt( norm, n, dl, d, du )

val = zlangt( norm, n, dl, d, du )

## Description

The routine returns the value of the 1-norm, or the Frobenius norm, or the infinity norm, or the element of largest absolute value of a real/complex tridiagonal matrix A.

## Input Parameters

norm

  CHARACTER*1. Specifies the vaule to be returned by the routine:

  = 'M' or 'm': val = max(abs(Ai j)), largest absolute value of the matrix A.

  = '1' or 'O' or 'o': val = norm1(A), 1-norm of the matrix A (maximum column sum),

  = 'I' or 'i': val = normI(A), infinity norm of the matrix A (maximum row sum),

  = 'F', 'f', 'E' or 'e': val = normF(A), Frobenius norm of the matrix A (square root of sum of squares).

n

  INTEGER. The order of the matrix A. `n ≥ 0`. When `n = 0`, ?langt is set to zero.

dl, d, du

  REAL for slangt

  DOUBLE PRECISION for dlangt

  COMPLEX for clangt

  COMPLEX*16 for zlangt

  Arrays: dl (n-1), d (n), du (n-1).

  The array dl contains the (n-1) sub-diagonal elements of A.

  The array d contains the diagonal elements of A.

  The array du contains the (n-1) super-diagonal elements of A.

## Output Parameters

val

  REAL for slangt/clangt

  DOUBLE PRECISION for dlangt/zlangt

  Value returned by the function.

Parent topic: Auxiliary Routines