1. a line in my code is something like int(phi*x^2,0,2) where i want phi to be a function which the user has choosen. But how can i let the user choose a function? For numbers you can use input but how to do it with a function? 2. i want to solve a nonlinear system of equations in x,y. exp(-2x)*(2*r*x-1)+1=8*pi*r^2 (1) exp(-2x)*(2*r*y+1)-1=8*pi*r^2 Do you have to specify r before you solve the system or can you get a solution as function of r, x=f(r) (2) y=g(r) ? x and y are parameters in a spherical symmetric metric and r are the radius so i guess you need a solution like (2)? But is that possible in matlab? Do you need to solve the system for many different r's and then plot the x and y as a function a r? Thank you.