# Forward and Backward Euler.

I want to consider this differential system: $$\ \frac{dx}{dt} = -y(t)\\ \frac{dy}{dt} = \ x(t)$$ where $t>0$ with initial condition$(x(0),y(0))=(1,0).$

First I want to show that this differential equation admits an invariant of $I = x(t)^2 + y(t)^2$ Also, Can someone help me to figure out if Forward Euler, backwards Euler, or implicit trapezoidal rule admit invariants similar to $I = x(t)^2 + y(t)^2$?

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Does anyone know how we can find that the diffeq admits that invariant that I described? –  Jackson Hart Nov 12 '12 at 21:08

I would like to explain this, using explicit Euler, because it is the most elementary. We have: \begin{align} \begin{pmatrix} x \\ y \end{pmatrix}' = \begin{pmatrix} -y \\ x \end{pmatrix} = f(x,y) \end{align} Explicit euler looks like this: $z_{k+1} = z_k +h \cdot f(z_k)$, applied to our problem it becomes: \begin{align} \begin{pmatrix} x_{k+1} \\ y_{k+1} \end{pmatrix} = \begin{pmatrix} x_{k} \\ y_{k} \end{pmatrix} +h \begin{pmatrix} -y_{k} \\ x_{k} \end{pmatrix} = \begin{pmatrix} x_{k}-hy_k \\ y_{k}+h x_k \end{pmatrix} \end{align} So if we want to know, whether $x(t)^2+y(t)^2$ are constant, we check \begin{align} x_{k+1}^2+y_{k+1}^2 = (x_k -h y_k)^2 +(y_k+ h x_k)^2 = (x_k^2+y_k^2)(1+h^2) \end{align} Give the initial values $x_0=1$ and $y_0=0$ and using recursion, we get \begin{align} x_{k+1}^2+y_{k+1}^2 &=(x_k^2+y_k^2)(1+h^2) =(x_{k-1}^2+y_{k-1}^2)(1+h^2)^2 =\ ... \ =(x_0^2+y_0^2)(1+h^2)^{k+1}\\ &= (1+h^2)^{k+1} \end{align} So it is no invariant.
Next implicit euler. The method reads $z_{k+1} = z_k +h \cdot f(z_{k+1})$ So we get \begin{align} & \begin{pmatrix} x_{k+1} \\ y_{k+1} \end{pmatrix} = \begin{pmatrix} x_{k} \\ y_{k} \end{pmatrix} +h \begin{pmatrix} -y_{k+1} \\ x_{k+1} \end{pmatrix} = \begin{pmatrix} x_{k}-hy_{k+1} \\ y_{k}+h x_{k+1} \end{pmatrix} \\ & \Leftrightarrow \begin{pmatrix} x_{k+1}+ h y_{k+1} \\ y_{k+1} -h x_{k+1} \end{pmatrix} = \begin{pmatrix} x_{k} \\ y_{k} \end{pmatrix} \\ & \Leftrightarrow \begin{pmatrix} 1 & h \\ 1 & -h \end{pmatrix} \begin{pmatrix} x_{k+1} \\ y_{k+1} \end{pmatrix} = \begin{pmatrix} x_{k} \\ y_{k} \end{pmatrix} \end{align} So as you can see, you end up with a linear system you have to solve for $x_{k+1}$ and $y_{k+1}$. This is the price you pay for an implicit scheme. If you solve for $x_{k+1}$ and $y_{k+1}$ you can again check the invariant. Good luck,
I showed that $x(t)^2+y(t)^2$ are not invariant for all $t$. If they where, also $x_{k+1}^2+y_{k+1}^2$ would be constant for all $k$. But this is not the case. For larger $k$ we get $x_{k+1}^2+y_{k+1}^2 \rightarrow \infty$ with $h>0$. –  sonystarmap Nov 12 '12 at 21:38