symbolic matrix norm

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It depends on what you call a "symbolic matrix". If M is a matrix with entries in the symbolic ring SR, then M has a method .norm() that computes a norm of M (you can specify which norm as a parameter). However, this method only works when the entries of the matrix can be converted to fixed real or complex numbers (elements of CDF or RDF).

for more details, type:

sage: M.norm?

for the documentation and

sage: M.norm??

for the source code of this method.

Hence, if you want to compute a norm (there are many possibilities) of a matrix with symbolic entries such as cos(x), then you have to write your own function.