differential eq with square root

[xapiens]

Hi!

I have to solve a system of two coupled nonlinear ordinary differential equations. In the function that defines the system there are terms with square root of the dependant variables. I tried to solve the system numerically but in the nmerical solution I get violent oscillations. Then I realized that the same problem appears in the numerical solution of this very simple exactly solvable differential equation: x'(t)=(1-x(t))^(1/2) with initial condition x(2.1)=0.9975. The exact solution is x(t)=(1/4)(4t-t^2).

It seems to me that the problem is the square root (1-x(t))^(1/2). When the numerical solution approach an extremum, then x'(t) and (1-x(t))^(1/2) both approach zero. But then (1-x(t)) also approaches zero, and in fact (1-x(t)) takes negative values, that makes the square root imaginary.

I explore the situation with the fourth-order Runge-Kutta method with step h=0.2 and in the firt iteration I find the complex value:

x(2.1)=1.00463+ 0.0042963 I

The same happens with the Euler method and the MidPoint Method

Any hint of how to get the correct numerical solution of this equation???

Thanks a lot!