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% Simple ODE system for double pendulum
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function dx_dt = f(t,x)
%Since there are 2 ODE of degree 2, you can expect that your state
%will be made of 4 variables. 

%shortcuts
x1 = x(1);	%theta1
x2 = x(2);	%theta1_dot
x3 = x(3);	%theta2
x4 = x(4);	%theta2_dot

%global variable: 
global g_L

%define your derivative here:
dx_dt = [];