norm {Matrix} | R Documentation

## Matrix Norms

### Description

Computes a matrix norm of `x`, using Lapack for dense matrices. The norm can be the one (`"O"`, or `"1"`) norm, the infinity (`"I"`) norm, the Frobenius (`"F"`) norm, the maximum modulus (`"M"`) among elements of a matrix, or the spectral norm or 2-norm (`"2"`), as determined by the value of `type`.

### Usage

    norm(x, type, ...)

### Arguments

`x` | a real or complex matrix.

`type`

A character indicating the type of norm desired.

`"O"`, `"o"` or `"1"`

  specifies the one norm, (maximum absolute column sum);

`"I"` or `"i"`

  specifies the infinity norm (maximum absolute row sum);

`"F"` or `"f"`

  specifies the Frobenius norm (the Euclidean norm of `x` treated as if it were a vector);

`"M"` or `"m"`

  specifies the maximum modulus of all the elements in `x`; and

`"2"`

  specifies the “spectral norm” or 2-norm, which is the largest singular value (`svd`) of `x`.

The default is `"O"`. Only the first character of `type[1]` is used.

`...` | further arguments passed to or from other methods.

### Details

For dense matrices, the methods eventually call the Lapack functions `dlange`, `dlansy`, `dlantr`, `zlange`, `zlansy`, and `zlantr`.

### Value

A numeric value of class `"norm"`, representing the quantity chosen according to `type`.

### References

Anderson, E., et al. (1994). LAPACK User's Guide, 2nd edition, SIAM, Philadelphia.

### See Also

`onenormest()`, an approximate randomized estimate of the 1-norm condition number, efficient for large sparse matrices.

The `norm()` function from R's base package.

### Examples

    x <- Hilbert(9)
    norm(x)# = "O" = "1"
    stopifnot(identical(norm(x), norm(x, "1")))
    norm(x, "I")# the same, because 'x' is symmetric

    allnorms <- function(d) vapply(c("1","I","F","M","2"),
                                   norm, x = d, double(1))
    allnorms(x)
    allnorms(Hilbert(10))

    i <- c(1,3:8); j <- c(2,9,6:10); x <- 7 * (1:7)
    A <- sparseMatrix(i, j, x = x)                      ##  8 x 10 "dgCMatrix"
    (sA <- sparseMatrix(i, j, x = x, symmetric = TRUE)) ## 10 x 10 "dsCMatrix"
    (tA <- sparseMatrix(i, j, x = x, triangular= TRUE)) ## 10 x 10 "dtCMatrix"
    (allnorms(A) -> nA)
    allnorms(sA)
    allnorms(tA)
    stopifnot(all.equal(nA, allnorms(as(A, "matrix"))),
    	  all.equal(nA, allnorms(tA))) # because tA == rbind(A, 0, 0)
    A. <- A; A.[1,3] <- NA
    stopifnot(is.na(allnorms(A.))) # gave error

[Package Matrix version 1.2-12 Index]