Says the vector is A, to perform fractional power of A, where the power is 0.2, type

B = A.^0.2

B is vector of the fractional power of A.

 In this file there are examples that you will not have any problem.

Thanks but they do not work in my case.The problem comes when A is negative. The answer would be of complex form  and that is not compatible with the rest of the system. take a dynamics system as an example, what if the control law involves fractional power of negative value?

It would be helpful if you can edit your question, by adding example of your code where leading to the error.

Dear Samira.

It's true if A is negative values,results will be complex numbers.There is one negative element in example . If you add your original file to run better.

However, sometimes some versions of MATLAB in Windows does not run correctly.

Dear Faezeh. Your file contained all possible solutions for the power fractions. Thanks alot. However, the system state is real and complex numbers are out of its domain. 

The solution is to take the absolute value of the variable and multiply it by a sign function to preserve the sign of the value. That is, if the variable is x, the power fraction should be of following form:

(abs(x)^p/q)*sign(x) where p and q are integer values and p/q is not an integer. I have edited the question to clarify the confusion.

I hope the final solution helps those encountering the same problem

Dear samira:

I hope this example will be useful.In this ,there are complex and real number and no error.

Dear Faezeh

Thank you