Spivak, Ch. 20: Understanding a step in the proof of lemma

In summary: Again, we use Cauchy MVT to write$$\frac{R(x)}{x-a}=\frac{R(x)-R(a)}{x-a}=R'(t)$$for some ##t\in (a,x)##. Then$$\frac{R(x)}{g(x)}=\frac{R(x)}{(x-a)^{k+2}}=\frac{R'(z)}{(k+2)(z-a)^{k+1}}=\frac{1}{k+2}\frac{R'(z)}{(z-a)^{k
  • #1
zenterix
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TL;DR Summary
In Chapter 20, "Approximation by Polynomial Functions", there is a lemma (used to prove Taylor's Theorem right after), the proof of which includes a step that I would like to understand better.
In Chapter 20 of Spivak's Calculus is the lemma shown below (used afterward to prove Taylor's Theorem). My question is about a step in the proof of this lemma.

Lemma: Suppose that the function ##R## is ##(n+1)##-times differentiable on ##[a,b]## and

$$R^{(k)}(a)=0, \text{ for } k=0,1,2,...,n$$

Then for any ##x## in ##(a,b]## we have

$$\frac{R(x)}{(x-a)^{n+1}}=\frac{R^{(n+1)}(t)}{(n+1)!}, \text{ for
some } t \text{ in } (a,x)\tag{1}$$

Here is the proof as it appears in the book

For ##n=0## this is just the Mean Value Theorem, and we will prove the
theorem for all ##n## by induction on ##n##. To do this we use the Cauchy
Mean Value Theorem to write

$$\frac{R(x)}{(x-a)^{n+2}}=\frac{R'(z)}{(n+2)(z-a)^{n+1}}=\frac{1}{n+2}\frac{R'(z)}{(z-a)^{n+1}},
\text{ for some } z \text{ in } (a,x)$$

and then apply the induction hypothesis to ##R'## on the interval
$[a,z]$ to get

$$\frac{R(x)}{(x-a)^{n+2}}=\frac{1}{n+2}\frac{(R')^{(n+1)}(t)}{(n+1)!},
\text{ for some } t \text{ in } (a,z)\tag{2}$$

$$=\frac{R^{(n+2)}(t)}{(n+2)!}$$

My question is: how do we know that ##(R')^{n+1}## is defined in ##(2)##?

Let me try to go through the proof in more steps.

For ##n=0## we have

$$\frac{R(x)}{x-a}=\frac{R(x)-R(a)}{x-a}=R'(t)$$

for some ##t\in (a,x)##, which is the MVT applied to ##R## on ##[a,x]##.

Now assume as our induction hypothesis that ##(1)## is true for some ##k##.

First, let's apply Cauchy MVT to the functions ##R(x)## and ##g(y)=(y-a)^{k+2}##.

$$\frac{R(x)-R(a)}{g(x)-g(a)}=\frac{R'(z)}{g'(z)}$$

for some ##z\in (a,x)##. Then

$$\frac{R(x)}{g(x)}=\frac{R(x)}{(x-a)^{k+2}}=\frac{R'(z)}{(k+2)(z-a)^{k+1}}=\frac{1}{k+2}\frac{R'(z)}{(z-a)^{k+1}}$$

At this point, we are to apply the induction hypothesis to ##R'## on ##[a,z]##. In the induction hypothesis, ##R## is assumed to be ##(n+1)##-times differentiable. Doesn't this mean that ##R'## is only ##n##-times differentiable?

How do we know that ##(R')^{n+1}## is defined in ##(2)##?
 
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  • #2
Looks like the answer is a silly oversight.

Since the induction is over ##n##, this includes the part about the function ##R## being ##(n+1)##-times differentiable.

After the inductive hypothesis, we want to prove the result for some ##k+1##. To do this we assume that the function ##R## is ##(n+2)##-times differentiable.
 
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FAQ: Spivak, Ch. 20: Understanding a step in the proof of lemma

What is Spivak's Chapter 20 about?

Spivak's Chapter 20 is about understanding a step in the proof of a lemma. It focuses on breaking down the reasoning and logic behind a specific step in a mathematical proof to gain a deeper understanding of the concept.

What is a lemma in mathematics?

A lemma is a proven statement or theorem that is used as a stepping stone to prove a larger, more complex theorem. Lemmas are often used to simplify a proof or provide a more general result that can be applied to multiple situations.

Why is it important to understand each step in a proof?

Understanding each step in a proof is important because it allows for a deeper understanding of the concept being proved. It also helps to identify any potential errors or gaps in the logic of the proof.

What techniques are used in understanding a step in the proof of a lemma?

Some techniques used in understanding a step in the proof of a lemma include breaking down the statement into smaller, more manageable parts, using examples and counterexamples, and considering alternative approaches to the proof.

How can understanding a step in the proof of a lemma improve problem-solving skills?

Understanding a step in the proof of a lemma can improve problem-solving skills by developing a deeper understanding of the underlying concepts and logic used in mathematical proofs. This can also help in identifying patterns and connections between different mathematical concepts, making it easier to solve more complex problems.

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