How to resolve the discrepancy

Oct 13, 2025 by wbrum.org 25 views

Introduction

Differentiation matrices are a powerful tool in numerical analysis, used to approximate the derivatives of functions. They are particularly useful in solving partial differential equations (PDEs) and other problems where the derivative of a function is required. However, when using differentiation matrices, it is not uncommon to encounter situations where two equivalent expressions yield different numerical results. In this article, we will explore the reasons behind this discrepancy and provide insights into how to resolve it.

What are differentiation matrices?

Differentiation matrices are a way of approximating the derivative of a function using a matrix. The basic idea is to represent the function as a vector of values at a set of points, and then use a matrix to compute the derivative of the function at those points. The matrix is designed such that when it is multiplied by the vector of function values, the result is an approximation of the derivative of the function.

Why do differentiation matrices work?

Differentiation matrices work by approximating the derivative of a function using a finite difference formula. The finite difference formula is a way of approximating the derivative of a function using the values of the function at a set of points. The matrix is designed such that when it is multiplied by the vector of function values, the result is an approximation of the derivative of the function.

Why do two equivalent expressions yield different numerical results?

There are several reasons why two equivalent expressions may yield different numerical results when using differentiation matrices. Some of the reasons include:

Rounding errors: When performing numerical computations, rounding errors can occur. These errors can cause the results of two equivalent expressions to differ.
Matrix multiplication: When multiplying matrices, the order of the matrices can affect the result. If the matrices are not multiplied in the correct order, the result can be different.
Numerical instability: Differentiation matrices can be numerically unstable, meaning that small changes in the input can result in large changes in the output.
Approximation errors: Differentiation matrices are approximations of the derivative of a function. As such, they can introduce errors into the computation.

How to resolve the discrepancy

To resolve the discrepancy between two equivalent expressions, you can try the following:

Check for rounding errors: Check if the results of the two expressions are close to each other, but not exactly equal. If this is the case, it may be due to rounding errors.
Check the matrix multiplication: Check if the matrices are being multiplied in the correct order. If not, try multiplying them in the correct order.
Check for numerical instability: Check if the differentiation matrix is numerically stable. If not, try using a different matrix or a different method.
Check for approximation errors: Check if the differentiation matrix is introducing errors into the computation. If so, try using a different matrix or a different method.

Example

Let's consider an example to illustrate the concept. Suppose we want to compute the derivative of the function f(x) = x^2 at x = 1 using a differentiation matrix. We can represent the function as a vector of values at a set of points, and then use a matrix to compute the derivative of the function at those points.

import numpy as np
def f(x):
return x**2
def D(x):
return np.array([[1, -1]])
x = np.array([1])
Df = np.dot(D(x), f(x))
print(Df)

Conclusion

In conclusion, differentiation matrices are a powerful tool in numerical analysis, used to approximate the derivatives of functions. However, when using differentiation matrices, it is not uncommon to encounter situations where two equivalent expressions yield different numerical results. By understanding the reasons behind this discrepancy and using the techniques outlined in this article, you can resolve the discrepancy and obtain accurate results.

References

[1] LeVeque, R. J. (2007). Finite difference methods for ordinary and partial differential equations: steady-state and time-dependent problems. SIAM.
[2] Strang, G. (2007). Linear algebra and its applications. Thomson Brooks/Cole.
[3] Trefethen, L. N. (2000). Spectral methods in MATLAB. SIAM.

Code

import numpy as np
def f(x):
return x**2
def D(x):
return np.array([[1, -1]])
x = np.array([1])
Df = np.dot(D(x), f(x))
print(Df)

Note

Q: What is the main reason why two equivalent expressions yield different numerical results when using differentiation matrices?

A: The main reason is due to rounding errors. When performing numerical computations, rounding errors can occur, causing the results of two equivalent expressions to differ.

Q: Can you provide an example of how rounding errors can occur when using differentiation matrices?

A: Yes, consider the following example:

Suppose we want to compute the derivative of the function f(x) = x^2 at x = 1 using a differentiation matrix. We can represent the function as a vector of values at a set of points, and then use a matrix to compute the derivative of the function at those points.

import numpy as np
def f(x):
return x**2
def D(x):
return np.array([[1, -1]])
x = np.array([1.0])
Df = np.dot(D(x), f(x))
print(Df)

However, if we change the value of x to 1.0000001, we get a different result:

x = np.array([1.0000001])
Df = np.dot(D(x), f(x))
print(Df)

This is because the value of x is being rounded to a different value, causing the result to differ.

Q: How can I avoid rounding errors when using differentiation matrices?

A: To avoid rounding errors, you can use a higher precision data type, such as numpy.float64 or numpy.float128. You can also use a library that provides high-precision arithmetic, such as mpmath.

Q: Can you provide an example of how to use a higher precision data type when using differentiation matrices?

A: Yes, consider the following example:

import numpy as np
def f(x):
return x**2
def D(x):
return np.array([[1, -1]])
x = np.array([1.0], dtype=np.float128)
Df = np.dot(D(x), f(x))
print(Df)

In this example, we define the function f(x) = x^2 and the differentiation matrix D(x). We then define the points at which to compute the derivative, using the np.float128 data type to avoid rounding errors.

Q: Can you provide an example of how to use a library that provides high-precision arithmetic when using differentiation matrices?

A: Yes, consider the following example:

import mpmath
def f(x):
return x**2
def D(x):
return mpmath.matrix([[1, -1]])
x = mpmath.mpf(1.0)
Df = mpmath.dot(D(x), f(x))
print(Df)

In this example, we define the function f(x) = x^2 and the differentiation matrix D(x). We then define the points at which to compute the derivative, using the mpmath library to provide high-precision arithmetic.

Q: Can you provide an example of how to check for numerical instability when using differentiation matrices?

A: Yes, consider the following example:

import numpy as np
def f(x):
return x**2
def D(x):
return np.array([[1, -1]])
x = np.array([1.0])
Df = np.dot(D(x), f(x))
if np.any(np.isnan(Df)):
print("Numerical instability detected")
else:
print("Numerical instability not detected")

In this example, we define the function f(x) = x^2 and the differentiation matrix D(x). We then define the points at which to compute the derivative, and compute the derivative using the differentiation matrix. We then check for numerical instability by checking if any of the values in the result are NaN (Not a Number).

Q: Can you provide an example of how to check for approximation errors when using differentiation matrices?

A: Yes, consider the following example:

import numpy as np
def f(x):
return x**2
def D(x):
return np.array([[1, -1]])
x = np.array([1.0])
Df = np.dot(D(x), f(x))
if np.any(np.abs(Df - np.diff(f(x))) > 1e-6):
print("Approximation error detected")
else:
print("Approximation error not detected")

In this example, we define the function f(x) = x^2 and the differentiation matrix D(x). We then define the points at which to compute the derivative, and compute the derivative using the differentiation matrix. We then check for approximation errors by comparing the result with the exact derivative of the function.