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Page: Sobolev Space

Definition (Sobolev Space). The Sobolev space $W^{k,p}(U)$ consists of all locally summable functions $u: U \to \mathbb{R}$ such that for each multi-index $\alpha$ with $|\alpha| \leq k$, the weak derivative $D^{\alpha}u$ exists and belongs to $L^p(U)$.

$$\|u\|_{W^{k,p}(U)} = \left( \sum_{|\alpha| \leq k} \int_U |D^\alpha u|^p \, dx \right)^{1/p}$$

A key tool in the study of Sobolev spaces is the mollifier, which allows approximation of $L^p$ functions by smooth functions.

Source
\begin{definition}[Compact Space]
A topological space $X$ is \textbf{compact} if every
open cover of $X$ has a finite subcover. That is, if
$X = \bigcup_{\alpha} U_{\alpha}$, then there exist
$U_{\alpha_1}, \dots, U_{\alpha_n}$ with
$X = \bigcup_{i=1}^{n} U_{\alpha_i}$.
\end{definition}
 
\begin{theorem}[Heine–Borel]
A subset $S \subseteq \mathbb{R}^n$ is compact if and
only if it is closed and bounded.
\end{theorem}
 
\begin{figure}[h]
\centering
\includegraphics[width=0.6\textwidth]{heine-borel}
\caption{Finite subcover of $[0,1]$}
\end{figure}
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Definition (Compact Space). A topological space $X$ is compact if every open cover of $X$ has a finite subcover. That is, if $X = \bigcup_{\alpha} U_{\alpha}$, then there exist $U_{\alpha_1}, \dots, U_{\alpha_n}$ with $X = \bigcup_{i=1}^{n} U_{\alpha_i}$.

Theorem (Heine–Borel)

A subset $S \subseteq \mathbb{R}^n$ is compact if and only if it is closed and bounded.

U₁ U₂ U₃ U₄ 0 1
Figure 1: Finite subcover of $[0,1]$
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Lebesgue Dominated Convergence
Real Analysis Measure Theory
Let $\{f_n\}$ be a sequence of measurable functions such that $f_n \to f$ pointwise a.e. If there exists an integrable function $g$ with $|f_n| \leq g$ a.e., then: $$\lim_{n \to \infty} \int f_n \, d\mu = \int f \, d\mu$$
Knowledge Gaps
Pull Requests 2 open
Add Cauchy–Schwarz proof #47 by Maria • 2h ago
Fix typo in MVT remark #46 by Jake • 5h ago
Add L'Hôpital's Rule #45 by Alex • merged

Add Cauchy–Schwarz proof

Open
Maria wants to merge 1 commit into main
Cauchy–Schwarz Inequality.md +12 −0
1414$$|\langle u, v \rangle| \leq \|u\| \cdot \|v\|$$
1515
16[proof]
17If $v = 0$, the result is trivial. Suppose $v \neq 0$.
18For any $t \in \mathbb{R}$, we have $\|u + tv\|^2 \geq 0$:
19$$0 \leq \|u\|^2 + 2t\langle u,v\rangle + t^2\|v\|^2$$
20Setting $t = -\frac{\langle u,v\rangle}{\|v\|^2}$ yields
21$$|\langle u,v\rangle|^2 \leq \|u\|^2\|v\|^2. \quad\square$$
22[/proof]
1 approval

Fix typo in MVT remark

Open
Jake wants to merge 1 commit into main
Mean Value Theorem.md +1 −1
2828[remark:Geometric Interpretation]
29The MVT says there is always a point on the curve where the tangent
29The MVT says there is always a point on the curve where the **tangent**
3030line is parallel to the secant through $(a, f(a))$ and $(b, f(b))$.
3131This has important consequences for monotonicity: if $f'(x) > 0$
3232on an interval, then $f$ is strictly increasing.
3333[/remark]
2 approvals

Add L'Hôpital's Rule

Merged
Alex merged 1 commit into main
L'Hôpital's Rule.md +8 −0
1[theorem:L'Hôpital's Rule]
2If $\lim_{x \to c} f(x) = \lim_{x \to c} g(x) = 0$ (or $\pm\infty$),
3and $g'(x) \neq 0$ near $c$, then:
4$$\lim_{x \to c} \frac{f(x)}{g(x)} = \lim_{x \to c} \frac{f'(x)}{g'(x)}$$
5provided the right-hand limit exists.
6[/theorem]
Pull request successfully merged and closed
3 approvals
# general
# analysis
# algebra
A Alex
M
Maria 2:34 PM

Can someone explain why \(\int e^x \, dx = e^x + C\)?

👍 2
J
Jake 2:36 PM

The derivative of \(e^x\) is itself! So the antiderivative is also \(e^x\). It's the only function with this property.

💡 3 👍 1
M 1 reply Maria: That makes sense! So for...
S
Sophie 1:15 PM

Working through Rudin Ch.3 - anyone else finding uniform convergence tricky?

🙋 5
L
Leo 1:22 PM

The key insight: \(\sup_{x \in S} |f_n(x) - f(x)| \to 0\) as \(n \to \infty\). The sup must go to zero, not just pointwise!

💡 4
R
Rachel 11:30 AM

Quick question: Is \(\mathbb{Z}[\sqrt{-5}]\) a UFD?

T
Tom 11:45 AM

No! Classic counterexample: \(6 = 2 \cdot 3 = (1+\sqrt{-5})(1-\sqrt{-5})\). Two different factorizations!

🤯 2 👍 3
A
Alex Yesterday

Hey! Did you see the new page on spectral theory? Really well written.

You Yesterday

Yes! I added a few clarifications to the proof. The \(\lambda\)-eigenspace section needed more detail.

Y
Thread
J
Jake

The derivative of \(e^x\) is itself! So the antiderivative is also \(e^x\). It's the only function with this property.

M
Maria 2:38 PM

That makes sense! So for \(e^{2x}\), it would be \(\frac{1}{2}e^{2x}\) because of chain rule?

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