Let [tex]y=\cos(2x)[/tex], with first two derivatives
[tex]Dy = -2\sin(2x)[/tex]
[tex]D^2y = -4\cos(2x)[/tex]
Consider the linear ODE
[tex]aD^2y + bDy + cy = 0[/tex]
Substitute [tex]y[/tex] and its derivatives and solve for the unknown coefficients [tex]a,b,c[/tex].
[tex]-4a\cos(2x) - 2b\sin(2x) + c\cos(2x) = 0[/tex]
[tex]\implies \begin{cases}-4a+c=0 \\ -2b=0 \end{cases}[/tex]
[tex]\implies b=0 \text{ and } c= 4a[/tex]
Then the minimal ODE with this solution is
[tex]aD^2y + 4ay = 0 \implies \boxed{D^2y + 4y = 0}[/tex]
