Numerical underflow problem - help about matrix logarithm needed please

Hi all, I am currently using Matlab to calculate the value of a
function

f = A*expm(B*n)*C

where A is a 1x3 vector, B is an upper triangular 3x3 matrix, n is a
scalar and C is a 3x1 vector. f is then a scalar and I take the log
of f.

PROBLEM: For some values of n, i.e. large n, f turns out to be zero.
This is because the values of the matrix expm(B*n) all become zero
(probably due to the diagonal elements of B being negative).

Is there any way of calculting the logarithm of f without having to
take the exponential of B*n?

i.e. if

log(f) = log(A) + B*n + log(C)

was true it would be nice!
(clearly not true, but just to get the idea of what I mean!)

Any help greatly appreciated

Thanks

Ronan
.

Relevant Pages

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