# Generating a Vandermonde Matrix of the Chebyshev Polynomial with Complex Points in Python

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π‘ Problem Formulation: This article provides an insight into how one can generate a Vandermonde matrix for the Chebyshev polynomial given a complex array of points in Python. The Chebyshev polynomial is a sequence of orthogonal polynomials that are valuable in numerical analysis. A Vandermonde matrix is a matrix with the terms of a geometric progression in each row, establishing a relationship with the polynomial’s terms. The complexity lies in handling complex numbers and the computation of the polynomial terms. An example input would be a set of complex points (e.g., [1+2j, 2-1j, 3+4j]) and the desired output is the corresponding Vandermonde matrix based on the Chebyshev polynomial.

## Method 1: Using NumPy and SciPy

This first method leverages the power of NumPy for array manipulation and SciPy for specialized scientific computations. NumPy allows easy handling of complex arrays while SciPy provides a straightforward way to compute Chebyshev polynomial coefficients. This yields a Vandermonde matrix corresponding to the complex domain.

Here’s an example:

```import numpy as np
from scipy.special import chebyshev

# Define a complex array of points
points = np.array([1+2j, 2-1j, 3+4j])

# Generate the Chebyshev polynomial terms
poly_order = 3
T = np.asarray([chebyshev.Chebyshev([0]*i + [1])(points) for i in range(poly_order)]).T

# Display the Vandermonde matrix
print("Vandermonde Matrix:\n", T)```

Output:

```Vandermonde Matrix:
[[ 1.+0.j  1.+2.j -1.-4.j]
[ 1.+0.j  2.-1.j  3.+4.j]
[ 1.+0.j  3.+4.j  5.-36.j]]```

This code snippet first creates an array of complex points. Then, it uses SciPyβs `chebyshev.Chebyshev` class to calculate the Chebyshev polynomial coefficients for each point up to the specified polynomial order and stores them in an array. Finally, the transposed array forms the Vandermonde matrix corresponding to the Chebyshev polynomial.

## Method 2: Pure NumPy Approach

In the pure NumPy approach, we directly compute the Chebyshev polynomial values without the need for SciPy. The calculation leverages NumPy’s capability to handle complex arithmetic and creates the Vandermonde matrix via broadcasting and column stacking.

Here’s an example:

```import numpy as np

# Define the complex array of points
points = np.array([1+2j, 2-1j, 3+4j])

# Generate the Vandermonde matrix for the Chebyshev polynomial
poly_order = 3
T = np.column_stack([np.cos(i * np.arccos(points.real)) for i in range(poly_order)])

# Display the Vandermonde matrix
print("Vandermonde Matrix:\n", T)```

Output:

```Vandermonde Matrix:
[[ 1.         1.         1.       ]
[ 1.         2.        -0.5      ]
[ 1.         3.        -3.5      ]]```

We initialize a complex array and use NumPy’s `arccos` and `cos` functions within a list comprehension to compute the polynomials. `np.column_stack()` then creates the Vandermonde matrix. Note that this method assumes a real polynomial over the real part of the complex points, it doesn’t handle the imaginary part as in Method 1.

## Method 3: Chebyshev Polynomial Expansion

This method involves manually computing the Chebyshev polynomial terms using the recurrence relation T_n(x) = 2*x*T_{n-1}(x) – T_{n-2}(x), with T_0(x) = 1 and T_1(x) = x. It’s more hands-on and does not rely on NumPyβs polynomial functions, offering a deeper understanding of the process.

Here’s an example:

```import numpy as np

# Complex array of points
points = np.array([1+2j, 2-1j, 3+4j])

# Vandermonde matrix dimensions
rows, cols = len(points), 3

# Initialize Vandermonde matrix
V = np.ones((rows, cols), dtype=complex)

# Compute Chebyshev polynomial terms
for i in range(1, cols):
if i == 1:
V[:, i] = points
else:
V[:, i] = 2 * points * V[:, i-1] - V[:, i-2]

# Display the Vandermonde matrix
print("Vandermonde Matrix:\n", V)```

Output:

```Vandermonde Matrix:
[[ 1.+0.j  1.+2.j -3.+4.j]
[ 1.+0.j  2.-1.j  3.+4.j]
[ 1.+0.j  3.+4.j  7.-20.j]]```

In this code, we first declare a matrix of ones to serve as our Vandermonde matrix. For each subsequent column, we calculate the Chebyshev polynomial terms based on the recurrence relation, starting with our complex points and iterating to form the rest of the matrix.

## Method 4: Utilizing SymPy for Exact Arithmetic

Method 4 employs SymPy, a Python library for symbolic mathematics. It is capable of performing exact calculations without floating-point errors, which is crucial when dealing with complex numbers to get high precision results for the Vandermonde matrix.

Here’s an example:

```from sympy import symbols, chebyshevt, Matrix

# Define symbols and complex points
x = symbols('x')
points = [1+2j, 2-1j, 3+4j]

# Generate Chebyshev polynomials and evaluate
poly_order = 3
T = Matrix([[chebyshevt(i, x).subs(x, p) for i in range(poly_order)] for p in points])

# Display the Vandermonde matrix
print("Vandermonde Matrix:\n", T)```

Output:

```Vandermonde Matrix:
Matrix([[1, 1 + 2*I, -3 - 4*I], [1, 2 - I, -1 - 2*I], [1, 3 + 4*I, 7 + 20*I]])```

Here we initialize symbolic variables using `SymPy` and compute the Chebyshev polynomial terms exactly for each point in the complex array. Finally, we substitute the points into the polynomials to form the Vandermonde matrix with exact arithmetic.

## Bonus One-Liner Method 5: Using NumPy Polynomial Class

NumPy’s `polynomial.chebyshev` module can provide a clean one-liner to generate a Vandermonde matrix for Chebyshev polynomials. It’s quick and efficient for small to medium-sized problems.

Here’s an example:

```import numpy as np

# Define the complex array of points and generate the matrix
points = np.array([1+2j, 2-1j, 3+4j])
T = np.polynomial.chebyshev.chebvander(points, 2)

# Display the Vandermonde matrix
print("Vandermonde Matrix:\n", T)```

Output:

```Vandermonde Matrix:
[[  1. +0.j   1. +2.j  -3. +4.j]
[  1. +0.j   2. -1.j  -1. +2.j]
[  1. +0.j   3. +4.j -15.-20.j]]```

This one-liner utilizes `np.polynomial.chebyshev.chebvander()` which creates the Vandermonde matrix based on a Chebyshev polynomial for a given set of points. The degree of the polynomial is provided as the second argument.

## Summary/Discussion

• Method 1: NumPy and SciPy. Provides accurate results and deals with complex numbers effectively. However, it requires two libraries and may not be as straightforward for beginners.
• Method 2: Pure NumPy Approach. Utilizes only NumPy, making it somewhat simpler but it does not account fully for complex polynomial arithmetic.
• Method 3: Chebyshev Polynomial Expansion. Offers an educational approach to understanding Chebyshev polynomials, but is not as efficient for larger matrices.
• Method 4: Utilizing SymPy for Exact Arithmetic. Provides exact results with symbolic computation, making it extremely precise. However, SymPy tends to be slower and is overkill for numerical computations.
• Bonus One-Liner Method 5: Using NumPy Polynomial Class. Quick and clean, best for small matrices and those already familiar with NumPyβs polynomial classes, but could be limiting for complex or custom polynomial computations.