Approximation Theory - Introduction to Scientific Computing

Overview¶

Computers can only perform arithmetic. To work with functions like $\sin(x)$ or solutions to differential equations, we need approximations. Polynomials are the natural choice:

Easy to evaluate (Horner’s method, barycentric formulas)
Easy to differentiate and integrate
Converge rapidly for smooth functions

The key insight from Trefethen’s Approximation Theory and Approximation Practice is that we can work in two dual spaces:

Representation	Description	Advantages
Value space	$f(x_0), f(x_1), \ldots, f(x_n)$	Direct sampling, differentiation matrices
Coefficient space	$c_0, c_1, \ldots, c_n$ in $p(x) = \sum c_k T_k(x)$	Integration, coefficient decay analysis

The DCT (Discrete Cosine Transform) converts between them in $O(n \log n)$ operations.

Why Chebyshev?¶

For polynomial interpolation on $[-1, 1]$ , node placement matters enormously:

Equally spaced nodes: Runge phenomenon—error grows exponentially!
Chebyshev nodes: Error decreases exponentially for smooth functions

Chebyshev polynomials $T_n(x) = \cos(n \arccos x)$ are optimal in a minimax sense.

Chapter Contents¶

Lagrange Interpolation

The interpolation problem and barycentric formulas for stable evaluation.

Chebyshev Polynomials

Optimal node placement, minimax properties, and defeating Runge’s phenomenon.

Values and Coefficients

The dual representations and the DCT/FFT transforms between them.

Differentiation Matrices

Computing derivatives in value space—spectral differentiation.

Integration

Quadrature via Chebyshev coefficients vs. trapezoidal rule.

Spectral Accuracy

Why smooth functions achieve exponential convergence—coefficient decay theory.

Learning Outcomes¶

After completing this chapter, you should be able to:

L7.1: State existence/uniqueness for polynomial interpolation.
L7.2: Construct Lagrange basis polynomials and use barycentric evaluation.
L7.3: Explain Runge’s phenomenon and why Chebyshev nodes avoid it.
L7.4: Define Chebyshev polynomials via $T_n(x) = \cos(n\arccos x)$ .
L7.5: Transform between values and coefficients using DCT.
L7.6: Construct and apply Chebyshev differentiation matrices.
L7.7: Integrate using Chebyshev coefficients.
L7.8: Relate coefficient decay to function smoothness.
L7.9: Explain spectral vs algebraic convergence rates.

Key Formulas¶

Concept	Formula
Chebyshev polynomial	$T_n(x) = \cos(n \arccos x)$
Chebyshev nodes	$x_k = \cos\left(\frac{k\pi}{n}\right)$ , $k = 0, \ldots, n$
Barycentric weights	$w_k = (-1)^k \cdot \begin{cases} 1/2 & k = 0, n \\ 1 & \text{otherwise} \end{cases}$
Values → Coefficients	DCT (Type I)
Coefficients → Values	Inverse DCT
Chebyshev integral	$\int_{-1}^{1} T_k(x)\,dx = \frac{2}{1-k^2}$ (even $k$ )
Coefficient decay	$\|c_k\| = O(k^{-p-1})$ if $f^{(p)}$ has bounded variation

Applications¶

Function approximation: Represent $\sin$ , $\exp$ , special functions
Spectral methods: Solve ODEs/PDEs with exponential accuracy
Quadrature: High-accuracy numerical integration
Root finding: Chebyshev companion matrix methods

Connection to Fourier¶

Under the substitution $x = \cos\theta$ :

Chebyshev polynomials become cosines: $T_n(\cos\theta) = \cos(n\theta)$
The DCT is essentially a real FFT
Chebyshev interpolation $\leftrightarrow$ cosine interpolation

This connection is why Chebyshev methods achieve “spectral accuracy”—the same exponential convergence as Fourier methods, but for non-periodic functions on $[-1, 1]$ .