Spectral Norm for Hermitian Matrices
DESCRIPTION:
Computes the spectral (2) norm or a real symmetric or complex Hermitian matrix given its eigenvalue decomposition.
USAGE: 
norm.eigen.Hermitian(x)
REQUIRED ARGUMENTS:
x:
an object of class "eigen.Hermitian" representing the eigenvalue decomposition of a real symmetric or complex Hermitian matrix.
VALUE:
A numeric value of class "norm" representing the spectral or 2 norm of the matrix underlying x. A copy of the call to "norm" is returned as an attribute.
DETAILS:
The spectral or 2 norm the matrix underlying x is equal to its eigenvalue of largest magnitude.
REFERENCES:
Golub, G., and Van Loan, C. F. (1989). Matrix Computations, 2nd edition, Johns Hopkins, Baltimore.
SEE ALSO:
eigen , norm.Matrix , norm.svd.Matrix
EXAMPLES: 
x <- Matrix( sample(-3:3, size = 9, replace = T), nrow = 3, ncol = 3)
x[row(x) > col(x)] <- t(x)[row(x) > col(x)]  # construct symmetric matrix
class(x) <- Matrix.class(x)
norm(eigen(x))