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```{try_on_binder}
```
```{code-cell}
:tags: [remove-cell]
:load: myst_code_init.py
```
# Tutorial: Reducing an LTI system using balanced truncation
Here we briefly describe the balanced truncation method,
for asymptotically stable LTI systems with an invertible {math}`E` matrix,
and demonstrate it on the heat equation example from
{doc}`tutorial_lti_systems`.
First, we import necessary packages, including
{class}`~pymor.reductors.bt.BTReductor`.
```{code-cell}
import matplotlib.pyplot as plt
import numpy as np
import scipy.sparse as sps
from pymor.models.iosys import LTIModel
from pymor.reductors.bt import BTReductor
plt.rcParams['axes.grid'] = True
```
Then we build the matrices
```{code-cell}
:load: heat_equation.py
```
and form the full-order model.
```{code-cell}
fom = LTIModel.from_matrices(A, B, C, E=E)
```
## Balanced truncation
As the name suggests,
the balanced truncation method consists of
finding a *balanced* realization of the full-order LTI system and
*truncating* it to obtain a reduced-order model.
The balancing part is based on the fact that a single LTI system has many
realizations.
For example, starting from a realization
```{math}
\begin{align}
E \dot{x}(t) & = A x(t) + B u(t), \\
y(t) & = C x(t) + D u(t),
\end{align}
```
another realization can be obtained by replacing {math}`x(t)` with
{math}`T \tilde{x}(t)` or by pre-multiplying the differential equation with an
invertible matrix.
In particular, there exist invertible transformation matrices
{math}`T, S \in \mathbb{R}^{n \times n}` such that the realization with
{math}`\tilde{E} = S^{\operatorname{T}} E T = I`,
{math}`\tilde{A} = S^{\operatorname{T}} A T`,
{math}`\tilde{B} = S^{\operatorname{T}} B`,
{math}`\tilde{C} = C T`
has Gramians {math}`\tilde{P}` and {math}`\tilde{Q}` satisfying
{math}`\tilde{P} = \tilde{Q} = \Sigma = \operatorname{diag}(\sigma_i)`,
where {math}`\sigma_i` are the Hankel singular values
(see {doc}`tutorial_lti_systems` for more details).
Such a realization is called balanced.
The truncation part is based on the controllability and observability energies.
The controllability energy {math}`E_c(x_0)` is the minimum energy (squared
{math}`\mathcal{L}_2` norm of the input) necessary to steer the system from the
zero state to {math}`x_0`.
The observability energy {math}`E_o(x_0)` is the energy of the output (squared
{math}`\mathcal{L}_2` norm of the output) for a system starting at the state
{math}`x_0` and with zero input.
It can be shown for the balanced realization
(and same for any other realization)
that,
if {math}`\tilde{P}` is invertible,
then
```{math}
E_c(x_0) = x_0 \tilde{P}^{-1} x_0, \quad
E_o(x_0) = x_0 \tilde{Q} x_0.
```
Therefore, states corresponding to small Hankel singular values are more
difficult to reach (they have a large controllability energy) and are difficult
to observe (they produce a small observability energy).
In this sense, it is then reasonable to truncate these states.
This can be achieved by taking as basis matrices
{math}`V, W \in \mathbb{R}^{n \times r}` the first {math}`r` columns of
{math}`T` and {math}`S`,
possibly after orthonormalization,
giving a reduced-order model
```{math}
\begin{align}
\hat{E} \dot{\hat{x}}(t)
& = \hat{A} \hat{x}(t) + \hat{B} u(t), \\
\hat{y}(t)
& = \hat{C} \hat{x}(t) + D u(t),
\end{align}
```
with
{math}`\hat{E} = W^{\operatorname{T}} E V`,
{math}`\hat{A} = W^{\operatorname{T}} A V`,
{math}`\hat{B} = W^{\operatorname{T}} B`,
{math}`\hat{C} = C V`.
It is known that the reduced-order model is asymptotically stable if
{math}`\sigma_r > \sigma_{r + 1}`.
Furthermore, it satisfies the {math}`\mathcal{H}_\infty` error bound
```{math}
\lVert H - \hat{H} \rVert_{\mathcal{H}_\infty}
\leqslant 2 \sum_{i = r + 1}^n \sigma_i.
```
Note that any reduced-order model (not only from balanced truncation) satisfies
the lower bound
```{math}
\lVert H - \hat{H} \rVert_{\mathcal{H}_\infty}
\geqslant \sigma_{r + 1}.
```
## Balanced truncation in pyMOR
To run balanced truncation in pyMOR, we first need the reductor object
```{code-cell}
bt = BTReductor(fom)
```
Calling its {meth}`~pymor.reductors.bt.GenericBTReductor.reduce` method runs the
balanced truncation algorithm. This reductor additionally has an `error_bounds`
method which can compute the a priori {math}`\mathcal{H}_\infty` error bounds
based on the Hankel singular values:
```{code-cell}
error_bounds = bt.error_bounds()
hsv = fom.hsv()
fig, ax = plt.subplots()
ax.semilogy(range(1, len(error_bounds) + 1), error_bounds, '.-')
ax.semilogy(range(1, len(hsv)), hsv[1:], '.-')
ax.set_xlabel('Reduced order')
_ = ax.set_title(r'Upper and lower $\mathcal{H}_\infty$ error bounds')
```
To get a reduced-order model of order 10, we call the `reduce` method with the
appropriate argument:
```{code-cell}
rom = bt.reduce(10)
```
Instead, or in addition, a tolerance for the {math}`\mathcal{H}_\infty` error
can be specified, as well as the projection algorithm (by default, the
balancing-free square root method is used).
The used Petrov-Galerkin bases are stored in `bt.V` and `bt.W`.
We can compare the magnitude plots between the full-order and reduced-order
models
```{code-cell}
w = np.logspace(-2, 8, 300)
fig, ax = plt.subplots()
fom.mag_plot(w, ax=ax, label='FOM')
rom.mag_plot(w, ax=ax, linestyle='--', label='ROM')
_ = ax.legend()
```
as well as Bode plots
```{code-cell}
fig, axs = plt.subplots(6, 2, figsize=(12, 24), sharex=True, constrained_layout=True)
fom.bode_plot(w, ax=axs)
_ = rom.bode_plot(w, ax=axs, linestyle='--')
```
Also, we can plot the magnitude plot of the error system,
which is again an LTI system.
```{math}
\begin{align}
\begin{bmatrix}
E & 0 \\
0 & \hat{E}
\end{bmatrix}
\begin{bmatrix}
\dot{x}(t) \\
\dot{\hat{x}}(t)
\end{bmatrix}
& =
\begin{bmatrix}
A & 0 \\
0 & \hat{A}
\end{bmatrix}
\begin{bmatrix}
x(t) \\
\hat{x}(t)
\end{bmatrix}
+
\begin{bmatrix}
B \\
\hat{B}
\end{bmatrix}
u(t), \\
y(t) - \hat{y}(t)
& =
\begin{bmatrix}
C & -\hat{C}
\end{bmatrix}
\begin{bmatrix}
x(t) \\
\hat{x}(t)
\end{bmatrix}.
\end{align}
```
```{code-cell}
err = fom - rom
_ = err.mag_plot(w)
```
and its Bode plot
```{code-cell}
_ = err.bode_plot(w)
```
Finally, we can compute the relative errors in different system norms.
```{code-cell}
print(f'Relative Hinf error: {err.hinf_norm() / fom.hinf_norm():.3e}')
print(f'Relative H2 error: {err.h2_norm() / fom.h2_norm():.3e}')
print(f'Relative Hankel error: {err.hankel_norm() / fom.hankel_norm():.3e}')
```
Download the code:
{download}`tutorial_bt.md`,
{nb-download}`tutorial_bt.ipynb`.