# Lec 38 | MIT 18.01 Single Variable Calculus, Fall 2007

TLDRThe transcript from an MIT OpenCourseWare lecture explores the concept of infinity and series through a block stacking experiment, illustrating convergence and divergence. The professor demonstrates a 'greedy algorithm' approach to stacking and transitions into discussing power series, their properties, and their convergence. The lecture introduces Taylor's formula, showing how functions like e^x, sin(x), and cos(x) can be represented as power series. The emphasis is on the flexibility and computational utility of power series in representing a wide array of functions, akin to how decimal expansions represent real numbers.

###### Takeaways

- π The lecture is part of a series discussing mathematical concepts, specifically focusing on series and their applications.
- ποΈ The professor uses a physical demonstration with blocks to illustrate the concept of convergence and divergence in series.
- π Through the block stacking example, the professor explains the idea of a 'greedy algorithm' where the optimal choice at each step is made.
- π€ The class is encouraged to think critically about the possibility of stacking blocks to reach a certain point, highlighting the importance of mathematical reasoning.
- π The professor derives a formula to calculate the center of mass for the block stacking scenario, relating it to the harmonic series and its divergence.
- π The concept of power series is introduced, explaining how they can represent functions in a way similar to how decimal expansions represent numbers.
- π The rules for differentiating and integrating power series are discussed, drawing parallels to the same operations performed on polynomials.
- π’ The importance of the radius of convergence for power series is highlighted, which determines the interval in which the series converges.
- π Taylor's formula is introduced as a method to express functions as power series, involving the derivatives of the function evaluated at a point.
- π The professor provides examples of well-known functions like e^x, sin(x), and cos(x) expressed as power series, emphasizing their utility in calculus.

###### Q & A

### What is the main topic discussed in the script?

-The main topic discussed in the script is the concept of infinity and the properties of power series in mathematics.

### What is the significance of the block stacking experiment described in the script?

-The block stacking experiment is used to illustrate the concept of divergence and convergence in the context of series, and to demonstrate the idea of infinity having different orders or rates of growth.

### What is a 'greedy algorithm' as mentioned in the script?

-A 'greedy algorithm' is a strategy used in computer science where the optimal choice at each stage is made with the hope of finding the overall optimal solution. In the context of the script, it refers to building the block stack from the top down to maximize the distance each block extends.

### How does the script explain the concept of convergence in the context of series?

-The script explains convergence by discussing the block stacking scenario and the harmonic series. It suggests that if a series converges, it means that the terms of the series approach zero and the series can be summed to a finite value.

### What is the 'radius of convergence' mentioned in the script?

-The 'radius of convergence' is a concept related to power series that defines the interval around the center point (usually x=0) within which the series converges to a finite value.

### How does the script relate the concept of infinity to the growth properties of functions?

-The script uses the example of stacking blocks to show that infinity can have different 'characters' or rates of growth, such as exponential growth, and relates this to the logarithmic curve which grows very slowly.

### What is the geometric series and why is it significant in the script?

-The geometric series is a power series of the form 1 + x + x^2 + x^3 + ..., which converges for |x| < 1. It is significant in the script as it serves as an example to introduce the concept of power series and their convergence properties.

### How does the script explain the manipulation of power series?

-The script explains that power series can be manipulated in similar ways to polynomials, including addition, multiplication, substitution, division, differentiation, and integration.

### What is Taylor's formula as discussed in the script?

-Taylor's formula is a method to express a function as an infinite sum of terms, which are calculated using the derivatives of the function evaluated at a specific point, typically x=0, and scaled by the corresponding power of x and factorial coefficients.

### How does the script use the exponential function as an example of a power series?

-The script provides the Taylor series expansion of the exponential function, e^x, which is 1/0! + x/1! + x^2/2! + x^3/3! + ..., demonstrating how the function can be represented as a power series with all terms contributing to its value.

###### Outlines

##### π Introduction to MIT OpenCourseWare and a Block Stacking Challenge

The script begins with an introduction to MIT OpenCourseWare, highlighting its mission to provide free, high-quality educational resources. The speaker invites viewers to support and explore additional materials on the platform. Transitioning to a classroom scenario, the professor presents a physical challenge involving stacking blocks in a way that defies balance, aiming to illustrate concepts related to series in mathematics. The challenge is to build a stack with blocks extending further left without falling, prompting a classroom vote on the feasibility of the task with a given number of blocks.

##### π Exploring Convergence and Divergence through a Block Balancing Act

The professor delves into the concept of convergence and divergence in the context of the block stacking challenge. He explains that the outcome of the challenge could reveal an interesting mathematical property: either there is a limit to how far the blocks can extend (indicating convergence), or there isn't a limit (suggesting divergence). The professor then demonstrates a strategy to stack the blocks, emphasizing the importance of starting from the top down, contrary to the usual bottom-up approach. This method, known as the greedy algorithm, aims to optimize at each step, leading to a successful but precarious stack of blocks.

##### π Mathematical Analysis of the Block Stacking Strategy

The script continues with a detailed mathematical analysis of the block stacking strategy. The professor introduces a thought experiment to calculate the center of mass for a stack of blocks, using it to determine the optimal placement of each subsequent block. He establishes a recurrence formula to calculate the center of mass for an increasing number of blocks, revealing a pattern that relates to the harmonic series. The professor then engages the audience in a discussion about the potential extent of the block stack, using the harmonic series to estimate the number of blocks needed to reach a certain distance.

##### π The Implications of Infinity and Logarithmic Growth in the Block Stacking Problem

The professor discusses the implications of infinity in the context of the block stacking problem, highlighting the logarithmic growth rate of the harmonic series. He points out that while the series diverges, indicating that theoretically, one could stack blocks infinitely far, the practical application requires an enormous number of blocks to achieve even a modest distance. The professor uses the example of stacking blocks across a room, calculating that it would require a number of blocks whose height would be twice the distance to the moon, emphasizing the slow growth of logarithmic functions.

##### π Transitioning to Power Series and the Geometric Series Example

The script transitions to a new topic: power series. The professor introduces the concept by revisiting the geometric series, which sums to 1/(1-x) for values of x less than 1. He provides a proof for this formula, demonstrating how multiplying the series by x and subtracting the original series from the result leads to the formula. The professor acknowledges that the proof is flawed when x equals 1, as it leads to an undefined state, but is valid for convergent series where x is less than 1.

##### π’ Understanding Convergence and the Radius of Convergence for Power Series

The professor explains the importance of convergence in power series, emphasizing that it allows for mathematical manipulations that would otherwise be invalid. He introduces the concept of the radius of convergence, a value that determines the interval within which a power series converges. The professor clarifies that while there are methods to calculate the radius of convergence, they will not be discussed in detail, as it will be evident for any given series. He notes that the series will converge exponentially fast within the radius of convergence and diverge when x is greater than the radius.

##### π€ Audience Questions and the Role of Power Series in Mathematics

The script includes an interactive segment where the professor addresses audience questions about the nature of the convergence and how to identify it. He also explains why the series converge exponentially fast, attributing it to the power nature of the terms involved. Another question prompts the professor to discuss the method of finding the radius of convergence, which he states will be evident in any given series without needing complex calculations. The professor then outlines the importance of power series in representing a wide range of functions, akin to how decimal expansions represent real numbers.

##### π§© Operations with Power Series and the Concept of Taylor's Formula

The professor discusses the operations that can be performed with power series, drawing parallels with polynomials. He mentions addition, multiplication, substitution, and division as operations that are analogous for both. The focus then shifts to differentiation and integration, which are particularly relevant for calculus. The professor outlines the rules for differentiating and integrating power series, providing formulas for these operations. He also introduces Taylor's formula, which is a method to express functions in terms of their derivatives evaluated at a point, typically zero, multiplied by powers of x and divided by factorials.

##### π Examples of Taylor Series for e^x, sin(x), and cos(x)

The script concludes with examples of Taylor series for the exponential function e^x, and the trigonometric functions sin(x) and cos(x). For e^x, the professor shows that all derivatives evaluated at zero are one, leading to a series of 1/n! times x^n. For sin(x) and cos(x), the series are expressed with alternating signs and powers of x, with sin(x) involving only odd powers and cos(x) involving even powers. The professor emphasizes the similarity of these formulas, suggesting that recognizing their patterns will aid in memorization. The session ends with a note on practicing differentiation with these series in future lessons.

###### Mindmap

###### Keywords

##### π‘Creative Commons license

##### π‘MIT OpenCourseWare

##### π‘Series (mathematics)

##### π‘Convergence

##### π‘Divergence

##### π‘Greedy algorithm

##### π‘Center of mass

##### π‘Harmonic series

##### π‘Power series

##### π‘Radius of convergence

##### π‘Taylor's formula

###### Highlights

Introduction to MIT OpenCourseWare and its mission to provide free educational resources.

Demonstration of a block stacking problem to introduce the concept of series and convergence.

Engaging the audience with a physical experiment to explore mathematical concepts.

Explanation of the 'greedy algorithm' in the context of block stacking.

Discussion on the difference between convergence and divergence in series.

Illustration of how to build a block structure from the top down for optimal results.

Introduction of the concept of center of mass in relation to the block stacking problem.

Derivation of a recurrence formula to calculate the center of mass for a stack of blocks.

Connection between the block stacking problem and the harmonic series.

Analysis of the arithmetic pattern and its relation to the harmonic series.

Voting activity to gauge audience understanding and engagement with the problem.

Visualization of the logarithmic growth pattern in the block stacking scenario.

Discussion on the practical implications and limitations of the block stacking problem.

Transition to the topic of power series and their significance in mathematics.

Introduction and explanation of the geometric series as a power series.

Proof of the formula for the geometric series and its limitations.

General rules for power series convergence and the concept of the radius of convergence.

Differentiation and integration of power series and their parallels with polynomials.

Taylor's formula and its role in expressing functions as power series.

Examples of Taylor series for common functions like e^x, sin(x), and cos(x).

###### Transcripts

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