%ode_test4.m   used by  sim_difeq.m
%    x is scalar independent variable
%    y is vector dependent variables
%   dy is vector first derivatives of dependent variables
function dy=ode_test4(x, y)
dy = zeros(4,1); % force a column vector
dy(1) = 1.0;  % y(1) = acceleration
dy(2) = y(1); % dv/dt = a,  y(2) is velocity
dy(3) = y(2); % dx/dt = v,  y(3) is position
dy(4) = y(3); % test 4th order
end