Approximation of $\sin x$ on $[-\pi, \pi]$ by orthogonal projection

Example 6.63 from Axler's Linear Algebra Done Right 🙏

Goal: compute an approximation to the sine function that improves upon the Taylor Polynomial approximation from calculus.

• Let $C[-\pi, \pi]$ denote the real inner product space of continuous real-valued functions on $[-\pi, \pi]$ with inner product $\langle f, g \rangle = \int_{-\pi}^{\pi} fg$.
• Let $v \in C[-\pi, \pi]$ be the function $v(x) = \sin x$
• Let $U$ denote the subspace $\mathcal{P}_{5}(\mathbb{R}) \subseteq C[-\pi, \pi]$ (the space of polynomials of degree at most $5$ with real coefficients)
• The goal is to choose $u \in U$ such that $\Vert v - u \Vert$ is as small as possible where

$$ \begin{aligned} \Vert v - u \Vert = \int_{-\pi}^{\pi} \vert \sin x - u(x) \vert^{2} dx. \end{aligned} $$

• The solution is given by the orthogonal projection of $v(x) = \sin x$ onto $U = \mathcal{P}_{5}(\mathbb{R})$

Approach:

• Have standard basis of $\mathcal{P}_{5}(\mathbb{R})$

$$ \begin{aligned} (v_{1}, v_{2}, v_{3}, v_{4}, v_{5}, v_{6}) = (1, x, x^{2}, x^{3}, x^{4}, x^{5}). \end{aligned} $$

• Compute orthonormal basis of $\mathcal{P}_{5}(\mathbb{R})$ by applying the Gram-Schmidt procedure to the basis above.
• The $k$th vector of the orthonormal basis is computed as:

$$ \begin{aligned} f_{k} &= v_{k} - \frac{\langle v_{k}, f_{1} \rangle}{\Vert f_{1} \Vert^{2}} f_{1} - \cdots - \frac{\langle v_{k}, f_{k - 1} \rangle}{\Vert f_{k - 1} \Vert^{2}} f_{k - 1} \\ e_{k} &= \frac{f_{k}}{\Vert f_{k} \Vert}. \end{aligned} $$

• Compute the orthogonal projection of $v(x) = \sin x$ onto $\mathcal{P}_{5}(\mathbb{R})$ by using the orthonormal basis found above and the formula:

$$ \begin{aligned} P_{U}v = \langle v, e_{1} \rangle e_{1} + \cdots + \langle v, e_{m} \rangle e_{m}. \end{aligned} $$

Installation

• Install with

uv venv
uv pip install .

Results

See code.

❯ python gs_poly_project.py
Creating orthonormal basis...
Projecting sin(x) onto orthonormal basis...
Projection: 0.00564311797634678*x**5 - 0.155271410633428*x**3 + 0.987862135574673*x
Largest relative error for x ∈ [3, π]: (x=3.0, error= 271.11)

The orthogonal projection (function) is given by (approximately):

$$ \begin{aligned} u(x) = 0.987862135574673 \cdot x - 0.155271410633428 \cdot x^{3} - 0.00564311797634678 \cdot x^{5} \end{aligned} $$

The exact solution (see sympy output) equals:

$$ \begin{aligned} & P_U v = \frac{3 x}{\pi^{2}} + \frac{5 \sqrt{14} \left(x^{3} - \frac{3 \pi^{2} x}{5}\right) \left(- \frac{5 \sqrt{14} \left(- \frac{2 \pi^{3}}{5} + 6 \pi\right)}{4 \pi^{\frac{7}{2}}} + \frac{5 \sqrt{14} \left(- 6 \pi + \frac{2 \pi^{3}}{5}\right)}{4 \pi^{\frac{7}{2}}}\right)}{4 \pi^{\frac{7}{2}}} \\ & + \frac{63 \sqrt{22} \left(- \frac{63 \sqrt{22} \left(- 120 \pi - \frac{8 \pi^{5}}{63} + \frac{40 \pi^{3}}{3}\right)}{16 \pi^{\frac{11}{2}}} + \frac{63 \sqrt{22} \left(- \frac{40 \pi^{3}}{3} + \frac{8 \pi^{5}}{63} + 120 \pi\right)}{16 \pi^{\frac{11}{2}}}\right) \left(x^{5} - \frac{3 \pi^{4} x}{7} - \frac{10 \pi^{2} \left(x^{3} - \frac{3 \pi^{2} x}{5}\right)}{9}\right)}{16 \pi^{\frac{11}{2}}} \end{aligned} $$

Plots

Sine function, orthogonal projection and Taylor and approximation on $[-\pi, \pi]$

Sine function, orthogonal projection and Taylor and approximation on $[2, \pi]$

Sine function, orthogonal projection and Taylor and approximation on $[3, \pi]$