I have a mathematica question that i've tried quite hard without success to solve. I'm sure this has something to do with not using mathematica since 2001.

I have a matrix which I know to be positive definite. The entries of the matrix might be complicated but they are all real. To find an expression for the square root of this matrix (i.e., SS = A) I'm trying :

In[1]= Ftemp = {{F11,F12,F13,0,0},{F21,F22,0,F24,0},{0,0,F33,0,F35},{0,0,0,F44,F45},{0,0,F53,F54,F55}}; MatrixPower[Ftemp, 1/2] Out[1]= MatrixPower[(F11 F12 F13 0 0 F21 F22 0 F24 0 0 0 F33 0 F35 0 0 0 F44 F45 0 0 F53 F54 F55 ),1/2] 

why is this? Also, is it possible to assign the attributes of the matrix above so mathematica knows it is positive definite and could thus simplify the resulting diagonalisation / square rooting?

Many thanks, Paul