Example Of Non-commutativity Of Limits

Exploring the Non-Commutativity of Limits: When Order Matters

The concept of limits is fundamental to calculus and analysis. We often encounter situations where we need to evaluate limits involving multiple variables or multiple limiting processes. A crucial, and sometimes counterintuitive, aspect of this is the non-commutativity of limits. This means that the order in which we take limits can significantly affect the final result. Understanding this non-commutativity is crucial for avoiding errors and developing a deeper intuition for the behavior of functions in multi-variable settings. This article delves into this fascinating concept, providing examples and explanations to illustrate when and why the order of limits matters.

Understanding Limits

Before we dive into non-commutativity, let's briefly review the basic concept of a limit. For a function f(x), we say that the limit of f(x) as x approaches a is L, written as:

lim_x→a f(x) = L

This means that as x gets arbitrarily close to a, f(x) gets arbitrarily close to L. This concept extends naturally to functions of multiple variables. For a function f(x, y), the limit as x approaches a and y approaches b is written as:

lim_{(x,y)→(a,b)} f(x, y) = L

This means that as both x and y approach a and b respectively, f(x, y) approaches L. The subtle but crucial point here is that we are considering both variables approaching their respective limits simultaneously. However, we can also consider taking the limits sequentially, one variable at a time.

Iterated Limits: A Sequential Approach

When dealing with functions of multiple variables, we can consider taking limits iteratively. This involves taking the limit with respect to one variable first, and then taking the limit with respect to the other variable using the result of the first limit. This process is called taking iterated limits. For instance, for a function f(x, y), we can first take the limit as x approaches a, and then take the limit of the resulting expression as y approaches b. This is denoted as:

lim_y→b [lim_x→a *f(x, y)]

Alternatively, we could reverse the order:

lim_x→a [lim_y→b *f(x, y)]

The key point is that, in general, these two iterated limits are not equal. This is where the non-commutativity of limits comes into play.

Examples of Non-Commutative Limits

Let's examine some examples that clearly demonstrate the non-commutativity of limits. These examples will highlight the importance of considering the order of the limiting process.

Example 1: A Classic Case

Consider the function:

f(x, y) = xy / (x² + y²)

Let's examine the iterated limits as x and y approach 0.

First, let's consider the limit as x approaches 0, holding y constant:

lim_x→0 *f(x, y) = lim_x→0 (xy / (x² + y²)) = 0

Now, let's take the limit of this result as y approaches 0:

lim_y→0 [lim_x→0 *f(x, y)] = lim_y→0 0 = 0

Now, let's reverse the order. First, let's take the limit as y approaches 0, holding x constant:

lim_y→0 *f(x, y) = lim_y→0 (xy / (x² + y²)) = 0

Then, let's take the limit of this result as x approaches 0:

lim_x→0 [lim_y→0 *f(x, y)] = lim_x→0 0 = 0

In this case, both iterated limits are equal to 0. However, this is not always the case, as we'll see in the next example.

Example 2: A Demonstrative Counter-Example

Consider the function:

f(x, y) = (x²y) / (x⁴ + y²)

Let's examine the iterated limits as x and y approach 0.

First, let's take the limit as x approaches 0, holding y constant:

lim_x→0 *f(x, y) = lim_x→0 (x²y) / (x⁴ + y²) = 0

Then, taking the limit as y approaches 0:

lim_y→0 [lim_x→0 *f(x, y)] = 0

Now, let's reverse the order. First, take the limit as y approaches 0, holding x constant:

lim_y→0 *f(x, y) = lim_y→0 (x²y) / (x⁴ + y²) = 0

Then, taking the limit as x approaches 0:

lim_x→0 [lim_y→0 *f(x, y)] = 0

Again, both iterated limits are equal to 0. This might lead one to believe that commutativity always holds. However, this is deceptive, and the next example proves that.

Example 3: The Crucial Difference

Let's consider a slightly modified version of the previous example:

*g(x, y) = x²y / (x⁴ + y²) *

If we approach (0,0) along the path y = mx², we have:

lim_x→0 g(x, mx²) = lim_x→0 (x² * mx²) / (x⁴ + (mx²)²) = lim_x→0 (mx⁴) / (x⁴ + m²x⁴) = m/(1+m²)

This limit depends on m. Consequently, the double limit lim_{(x,y)→(0,0)} g(x, y) does not exist.

Let's consider the iterated limits:

1. Limit as x approaches 0 first: lim_x→0 g(x, y) = lim_x→0 x²y / (x⁴ + y²) = 0 for y ≠ 0. Therefore, lim_y→0 [lim_x→0 g(x, y)] = 0

2. Limit as y approaches 0 first: lim_y→0 g(x, y) = lim_y→0 x²y / (x⁴ + y²) = 0 for x ≠ 0. Therefore, lim_x→0 [lim_y→0 g(x, y)] = 0

Even though the iterated limits appear to commute in this case, the crucial point is that the double limit does not exist. This is a critical distinction. The existence of the double limit is a stronger condition than the equality of iterated limits.

Example 4: A More Explicit Demonstration of Non-Commutativity

Consider the function:

h(x,y) = x/(x+y)

Let's consider the iterated limits as x approaches 0 and y approaches 0.

• lim_x→0 h(x,y) = 0 for y≠0

• lim_y→0 [lim_x→0 h(x,y)] = 0

• lim_y→0 h(x,y) = 1 for x≠0

• lim_x→0 [lim_y→0 h(x,y)] = 1

Here, we clearly see that the order of the limits matters; the iterated limits yield different values. This explicitly showcases the non-commutativity.

Implications and Further Considerations

The non-commutativity of limits has important consequences in various areas of mathematics and its applications. For example:

• Multivariable Calculus: Understanding the non-commutativity of limits is essential for correctly evaluating partial derivatives and integrals of functions of multiple variables.
• Probability Theory: In probability theory, the concept is related to the order of taking conditional expectations and limits.
• Physics and Engineering: The non-commutativity of limits plays a role in the study of dynamical systems and other areas where limiting processes are crucial.

Furthermore, the existence of the iterated limits does not guarantee the existence of the double limit. As seen in Example 3, the iterated limits could exist and be equal, but the double limit might not exist. This emphasizes the significance of carefully analyzing the function's behavior near the limit point.

Frequently Asked Questions (FAQ)

Q1: Why does the order of limits matter?

The order of limits matters because the limiting process can affect how the function behaves. When we take the limit with respect to one variable first, we are essentially fixing the other variable, influencing the behavior of the function as the first variable approaches its limit. This influence can be lost if we reverse the order.

Q2: Is there a way to predict when limits will not commute?

There's no single, universal rule to predict when limits will not commute. However, functions with discontinuities or singularities near the limit point are often prime candidates for exhibiting this non-commutativity. Functions involving ratios where the denominator approaches zero along different paths frequently demonstrate this property.

Q3: How can I avoid making mistakes with non-commutative limits?

The best way to avoid errors is to carefully analyze the function's behavior near the limit point. Consider approaching the limit along different paths or calculating the iterated limits in both orders. If the results differ, it indicates non-commutativity.

Q4: What is the significance of the double limit?

The double limit, lim_{(x,y)→(a,b)} f(x,y), represents the simultaneous approach of x to 'a' and y to 'b'. Its existence is a stronger condition than the existence and equality of iterated limits. If the double limit exists, then the iterated limits will exist and be equal to it, but the converse is not always true.

Conclusion

The non-commutativity of limits is a subtle yet crucial concept in calculus and analysis. While the examples presented here highlight this non-commutativity, it is essential to remember that the existence of iterated limits does not guarantee the existence or equality of the double limit. A thorough understanding of this concept is vital for correctly evaluating limits involving multiple variables and avoiding potential errors in mathematical calculations and their applications in various fields. Always consider the order of limits, analyze the function's behavior carefully, and examine the existence of the double limit to ensure accuracy and avoid misinterpretations. This deeper understanding will equip you to handle limit problems with greater confidence and accuracy.