lu {Matrix} | R Documentation

## (Generalized) Triangular Decomposition of a Matrix

### Description

Computes (generalized) triangular decompositions of square and other dense matrices.

### Usage

    lu(x, ...)
    ## S4 method for signature 'matrix'
    lu(x, warnSing = TRUE, ...)
    ## S4 method for signature 'dgeMatrix'
    lu(x, warnSing = TRUE, ...)
    ## S4 method for signature 'dgCMatrix'
    lu(x, errSing = TRUE, order = TRUE, tol = 1, ...)

### Arguments

`x` | a dense or sparse matrix, in the latter case of square dimension. No missing values or IEEE special values are allowed.
`warnSing` | (when `x` is a `"denseMatrix"`) logical specifying if a `warning` should be signalled when `x` is singular.
`errSing` | (when `x` is a `"sparseMatrix"`) logical specifying if an error (see `stop`) should be signalled when `x` is singular. When `x` is singular, `lu(x, errSing=FALSE)` returns `NA` instead of an LU decomposition. No warning is signalled and the useR should be careful in that case.
`order` | logical or integer, used to chose which will-reducing permutation technique will be used internally. Do not change unless you know what you are doing.
`tol` | positive number indicating the pivoting tolerance used in `cs_lu`. Do only change with much care.
`...` | further arguments passed to or from other methods.

### Details

`lu()` is a generic function with special methods for different types of matrices. Use `showMethods("lu")` to list all the methods for the `lu` generic.

The method for class `dgeMatrix` (and all dense matrices) is based on LAPACK's `"dgetrf"` subroutine. It returns a decomposition also for singular and non-square matrices.

The method for class `dgCMatrix` (and all sparse matrices) is based on functions from the CSparse library. It signals an error (or returns `NA`, when `errSing = FALSE`, see above) when the decomposition algorithm fails, as when `x` is (too close to) singular.

### Value

An object of class `"LU"`, i.e., `"denseLU"` (see its separate help page), or `"sparseLU"`, see `sparseLU`; this is a representation of a triangular decomposition of `x`.

### References

Golub, G., and Van Loan, C. F. (1989). Matrix Computations, 2nd edition, Johns Hopkins, Baltimore.

Tim Davis (2005) http://www.cise.ufl.edu/research/sparse/CSparse/

Timothy A. Davis (2006) Direct Methods for Sparse Linear Systems, SIAM Series “Fundamentals of Algorithms”.

### See Also

Class definitions `LU` and `sparseLU` and function `expand`; `qr`, `chol`.

### Examples

    ##--- Dense  -------------------------
    x <- Matrix(rnorm(9), 3, 3)
    lu(x)
    dim(x2 <- round(10 * x[,-3]))# non-square
    expand(lu2 <- lu(x2))

    ##--- Sparse (see more in ?"sparseLU-class")----- % ./sparseLU-class.Rd

    pm <- as(readMM(system.file("external/pores_1.mtx",
                                package = "Matrix")),
             "CsparseMatrix")
    str(pmLU <- lu(pm))		# p is a 0-based permutation of the rows
                                    # q is a 0-based permutation of the columns
    ## permute rows and columns of original matrix
    ppm <- pm[pmLU@p + 1L, pmLU@q + 1L]
    pLU <- drop0(pmLU@L %*% pmLU@U) # L %*% U -- dropping extra zeros
    ## equal up to "rounding"
    ppm[1:14, 1:5]
    pLU[1:14, 1:5]

[Package Matrix version 1.0-6 Index]