\name{triang}


\alias{triang}
\alias{Triang}


\title{Upper and Lower Triangular Matrixes}


\description{
    
    Extracs the pper or lower tridiagonal part from a matrix.    

}


\usage{
triang(x)
Triang(x)
}


\arguments{

    \item{x}{
        a numeric matrix.
        }
        
}


\details{
    
    The functions \code{triang} and \code{Triang} allow to transform a
    square matrix to a lower or upper triangular form. 
    A triangular matrix is either an upper triangular matrix or lower 
    triangular matrix. For the first case all matrix elements \code{a[i,j]}
    of matrix \code{A} are zero for \code{i>j}, whereas in the second case
    we have just the opposite situation. A lower triangular matrix is 
    sometimes also called left triangular. In fact, triangular matrices 
    are so useful that much computational linear algebra begins with 
    factoring or decomposing a general matrix or matrices into triangular 
    form. Some matrix factorization methods are the Cholesky factorization 
    and the LU-factorization. Even including the factorization step, 
    enough later operations are typically avoided to yield an overall 
    time savings. Triangular matrices have the following properties: the 
    inverse of a triangular matrix is a triangular matrix, the product of 
    two triangular matrices is a triangular matrix, the determinant of a 
    triangular matrix is the product of the diagonal elements, the 
    eigenvalues of a triangular matrix are the diagonal elements.
    
}


\references{

Higham, N.J., (2002);
    \emph{Accuracy and Stability of Numerical Algorithms}, 
    2nd ed., SIAM.

Golub, van Loan, (1996);
    \emph{Matrix Computations}, 
    3rd edition. Johns Hopkins University Press. 
  
}


\examples{
## Create Pascal Matrix:
   P = pascal(3)
   P
   
## Create lower triangle matrix
   L = triang(P)
   L                                
}


\keyword{math}