function dx = lotkavolterra(t,x)
a = 1;  % preys' growth rate  
b = 1;    % prey's hunting rate
c = 1;   % predators' mortality rate 
d = 1;  % predators' hunting rate
dx = zeros(2,1); 
dx(1) = a * x(1) - b * x(1) * x(2) ;
dx(2) = -c*x(2)+ d*x(1)*x(2); 
end
