MATLAB: Speed optimization of partial inner product (norm)

Question

For a row vector, the norm can be written as

sqrt(sum(P.^2))

or

sqrt(P*P')

The latter is about twice as fast. Now I have a 4D matrix with dimensions [100,100,100,70], and would like to take the norm of the last dimension to yield a matrix of dimension [100,100,100]. This works:

sqrt(sum(P.^2,4))

but is too slow. Does anyone know a way to speed this up (perhaps in a similar way as the 1D case?)

Answer 1

This may help

http://www.mathworks.com/matlabcentral/fileexchange/29035-dnorm2