diff --git a/docs/math/real-analysis/2-reals.md b/docs/math/real-analysis/2-reals.md new file mode 100644 index 0000000..9d04547 --- /dev/null +++ b/docs/math/real-analysis/2-reals.md @@ -0,0 +1,67 @@ +# Chapter 2 + +## Section 2.1 - The Algebraic and Order Properties of Real Numbers + +**Proposition**. 2.1.1: $\mathbb{R}$ is a field, with zero element $0$ and identity $1$. + +**Definition**. The rational numbers $\mathbb{Q}$ is the field of fractions of the natural numbers $\mathbb{N}$. + +**Theorem**. 2.1.4: There does not exist a rational number $r$ such that $r^2 = 2$. + +--- + +**Definition**. An *ordered field* is a field $F$ together with subset $F^+$ such that + +1. $F+$ is closed under addition and multiplication +2. If $a \in F$, then exclusively $a \in F^+$, $a = 0$, or $-a \in F^+$. + +**Theorem**. In any ordered field $F$, the following hold + +1. $1 \in F^+$ +2. $\mathbb{N} \subset F^+$ +3. If $a \in F^+$, then $\frac{1}{a} \in F^+$ + +**Definition** The order relation $a > b$ and $b < a$ is defined by $a - b \in F^+$. + +**Theorem**. If $a, b, c \in F$, then + +1. One of $a > b$, $a = b$, or $a < b$ hold (trichotomy) +2. If $a > b$ and $b > c$, then $a > c$ (transitivity) +3. If $a > b$, then $-a < -b$ +4. If $a > b$ and $c > 0$, then $ac > bc$ +5. If $a > b$ and $c < 0$, then $ac < bc$ +6. If $a > b > 0$, then $\frac{1}{b} > \frac{1}{a} > 0$ + +--- + +**Definition**. Let $S$ be a nonempty subset of ordered field $F$. Then, $S$ is *bounded above* if there exists some $u \in F$ such that $s \leq u$ for all $s \in S$. Then, said element $u$ is an *upper bound* of $S$. + +**Definition**. Let $S$ be a nonempty subset of ordered field $F$. Then, $S$ is *bounded below* if there exists some $u \in F$ such that $s \geq u$ for all $s \in S$. Then, said element $u$ is a *lower bound* of $S$. + +**Definition**. Let $S$ be a nonempty subset of ordered field $F$. Then, $S$ is *bounded* if it is bounded both above and below. + +**Definition**. Given field $F$ and nonempty subset $S \subset F$, an element $u \in F$ is a *supremum* or *least upper bound* of $S$ if $u$ is an upper bound of $S$, and given any other upper bound $v$, then $u < v$ + +**Definition**. Given field $F$ and nonempty subset $S \subset F$, an element $u \in F$ is an *infimum* or *greatest lower bound* of $S$ if $u$ is a lower bound of $S$, and given any other lower bound $v$, then $u > v$ + +**Definition**. Given an ordered field $F$, the field has the *supremum/infimum property* if given any nonempty subset $S$, if $S$ is bounded above/below, $S$ has a supremum/infimum. + +## Section 2.2 - Absolute Value and the Real Line + +**Definition**. Absolute value is defined as normal (piecewise). Multiline function in LaTeX are hard. + +**Theorem**. Given any $a, b \in \mathbb{R}$, we know that + +1. $|a| > 0$ for $a \neq 0$ +2. $|ab| = |a||b|$ +3. $|a + b| \leq |a| + |b|$ + +**Collary**. Givem $a, b \in \mathbb{R}$, then $\abs{\abs{a} - \abs{b}} \leq \abs{a - b}$. + +**Remark**. Every field has at least one absolute value function. + +**Theorem**. In an ordered field $F$, for any $r > 0$, we know that + +1. $\abs x = r$ if and only if $x = r$ or $x = -r$ +2. $\abs x < r$ if and only if $-r < x < r$ +3. $\abs x > r$ if either $x > r$ or $x < -r$ diff --git a/docs/physics/electrostatics/Homework 5.rnote b/docs/physics/electrostatics/Homework 5.rnote new file mode 100644 index 0000000..76f8d67 Binary files /dev/null and b/docs/physics/electrostatics/Homework 5.rnote differ diff --git a/docs/physics/electrostatics/Homework5.rnote b/docs/physics/electrostatics/Homework5.rnote new file mode 100644 index 0000000..bc2e4f8 Binary files /dev/null and b/docs/physics/electrostatics/Homework5.rnote differ