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sav08:sets_and_relations [2010/02/26 22:03] vkuncak |
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- | ====== Sets and Relations ====== | ||

- | |||

- | ===== Sets ===== | ||

- | |||

- | Sets are unordered collection of elements. | ||

- | |||

- | We denote a finite set containing only elements $a$, $b$ and $c$ by $\{ a, b, c \}$. The order and number of occurrences does not matter: | ||

- | $\{ a, b, c \} = \{ c, a, b \} = \{ a, b, b, c \}$. | ||

- | * $a \in \{ a,b,c \}$ | ||

- | * $d \notin \{a,b,c\}$ iff $d \neq a \land d \neq b \land d \neq c$ | ||

- | |||

- | Empty set: $\emptyset$. For every $x$ we have $x \notin \emptyset$. | ||

- | |||

- | To denote large or infinite sets we can use set comprehensions: $\{ x.\ P(x) \}$ is set of all objects with property $P$. | ||

- | \[ | ||

- | y \in \{ x. P(x) \} \ \leftrightarrow\ P(y) | ||

- | \] | ||

- | |||

- | Notation for set comprehension: $\{ f(x)|x. P(x) \} = \{ y. (\exists x. y=f(x) \land P(x)) \}$ | ||

- | |||

- | Sometimes the binder $x$ can be inferred from context so we write simply $\{ f(x) | P(x) \}$. In general there is ambiguity in which variables are bound. (Example: what does the $a$ in $f(a,b)$ refer to in the expression: | ||

- | \[ | ||

- | \{a \} \cup \{ f(a,b) \mid P(a,b) \} | ||

- | \] | ||

- | does it refer to the outerone $a$ as in $\{a\}$ or is it a newly bound variable? The notation with dot and bar resolves this ambiguity. | ||

- | |||

- | Subset: $A \subseteq B$ means $\forall x. x \in A \rightarrow x \in B$ | ||

- | |||

- | \[ | ||

- | A \cup B = \{ x. x \in A \lor x \in B \} | ||

- | \] | ||

- | \[ | ||

- | A \cap B = \{ x. x \in A \land x \in B \} | ||

- | \] | ||

- | \[ | ||

- | A \setminus B = \{ x. x \in A \land x \notin B \} | ||

- | \] | ||

- | |||

- | Boolean algebra of subsets of some set $U$ (we define $A^c = U \setminus A$): | ||

- | * $\cup, \cap$ are associative, commutative, idempotent | ||

- | * neutral and zero elements: $A \cup \emptyset = A$, $A \cap \emptyset = \emptyset$ | ||

- | * absorption: $A \cup A = A$, $A \cap A = A$ | ||

- | * deMorgan laws: $(A \cup B)^c = A^c \cap B^c$, $(A \cap B)^c = A^c \cup B^c$ | ||

- | * complement as partition of universal set: $A \cap A^c = \emptyset$, $A \cup A^c = U$ | ||

- | * double complement: $(A^c)^c = A$ | ||

- | |||

- | Which axioms are sufficient? | ||

- | * [[http://citeseer.ist.psu.edu/57459.html|William McCune: Solution of the Robbins Problem. J. Autom. Reasoning 19(3): 263-276 (1997)]] | ||

- | |||

- | |||

- | |||

- | |||

- | |||

- | ==== Infinte Unions and Intersections ==== | ||

- | |||

- | Note that sets can be nested. Consider, for example, the following set $S$ | ||

- | \[ | ||

- | S = \{ \{ p, \{q, r\} \}, r \} | ||

- | \] | ||

- | This set has two elements. The first element is another set. We have $\{ p, \{q, r\} \} \in S$. Note that it is not the case that | ||

- | |||

- | Suppose that we have a set $B$ that contains other sets. We define union of the sets contained in $B$ as follows: | ||

- | \[ | ||

- | \bigcup B = \{ x.\ \exists a. a \in B \land x \in a \} | ||

- | \] | ||

- | As a special case, we have | ||

- | \[ | ||

- | \bigcup \{ a_1, a_2, a_3 \} = a_1 \cup a_2 \cup a_3 | ||

- | \] | ||

- | Often the elements of the set $B$ are computed by a set comprehension of the form $B = \{ f(i).\ i \in J \}$. | ||

- | We then write | ||

- | \[ | ||

- | \bigcup_{i \in J} f(i) | ||

- | \] | ||

- | and the meaning is | ||

- | \[ | ||

- | \bigcup \{ f(i).\ i \in J \} | ||

- | \] | ||

- | Therefore, $x \in \bigcup \{ f(i).\ i \in J \}$ is equivalent to $\exists i.\ i \in J \land x \in f(i)$. | ||

- | |||

- | We analogously define intersection of elements in the set: | ||

- | \[ | ||

- | \bigcap B = \{ x. \forall a. a \in B \rightarrow x \in a \} | ||

- | \] | ||

- | As a special case, we have | ||

- | \[ | ||

- | \bigcap \{ a_1, a_2, a_3 \} = a_1 \cap a_2 \cap a_3 | ||

- | \] | ||

- | We similarly define intersection of an infinite family | ||

- | \[ | ||

- | \bigcap_{i \in J} f(i) | ||

- | \] | ||

- | and the meaning is | ||

- | \[ | ||

- | \bigcap \{ f(i).\ i \in J \} | ||

- | \] | ||

- | Therefore, $x \in \bigcap \{ f(i).\ i \in J \}$ is equivalent to $\forall i.\ i \in J \rightarrow x \in f(i)$. | ||

- | |||

- | |||

- | ===== Relations ===== | ||

- | |||

- | Pairs: | ||

- | \[ | ||

- | (a,b) = (u,v) \iff (a = u \land b = v) | ||

- | \] | ||

- | Cartesian product: | ||

- | \[ | ||

- | A \times B = \{ (x,y) \mid x \in A \land y \in B \} | ||

- | \] | ||

- | |||

- | Relations $r$ is simply a subset of $A \times B$, that is $r \subseteq A \times B$. | ||

- | |||

- | Note: | ||

- | \[ | ||

- | A \times (B \cap C) = (A \times B) \cap (A \times C) | ||

- | \] | ||

- | \[ | ||

- | A \times (B \cup C) = (A \times B) \cup (A \times C) | ||

- | \] | ||

- | |||

- | === Diagonal relation === | ||

- | |||

- | $\Delta_A \subseteq A \times A$, is given by | ||

- | \[ | ||

- | \Delta_A = \{(x,x) \mid x \in A\} | ||

- | \] | ||

- | |||

- | ==== Set operations ==== | ||

- | |||

- | Relations are sets of pairs, so operations $\cap, \cup, \setminus$ apply. | ||

- | |||

- | ==== Relation Inverse ==== | ||

- | |||

- | \[ | ||

- | r^{-1} = \{(y,x) \mid (x,y) \in r \} | ||

- | \] | ||

- | |||

- | ==== Relation Composition ==== | ||

- | |||

- | \[ | ||

- | r_1 \circ r_2 = \{ (x,z) \mid \exists y. (x,y) \in r_1 \land (y,z) \in r_2\} | ||

- | \] | ||

- | |||

- | Note: relations on a set $A$ together with relation composition and $\Delta_A$ form a //monoid// structure: | ||

- | \[ | ||

- | \begin{array}{l} | ||

- | r_1 \circ (r_2 \circ r_3) = (r_1 \circ r_2) \circ r_3 \\ | ||

- | r \circ \Delta_A = r = \Delta_A \circ r | ||

- | \end{array} | ||

- | \] | ||

- | |||

- | Moreover, | ||

- | \[ | ||

- | \emptyset \circ r = \emptyset = r \circ \emptyset | ||

- | \] | ||

- | \[ | ||

- | r_1 \subseteq r_2 \rightarrow r_1 \circ s \subseteq r_2 \circ s | ||

- | \] | ||

- | \[ | ||

- | r_1 \subseteq r_2 \rightarrow s \circ r_1 \subseteq s \circ r_2 | ||

- | \] | ||

- | |||

- | |||

- | |||

- | ==== Relation Image ==== | ||

- | |||

- | When $S \subseteq A$ and $r \subseteq A \times A$ we define **image** of a set $S$ under relation $A$ as | ||

- | \[ | ||

- | S\bullet r = \{ y.\ \exists x. x \in S \land (x,y) \in r \} | ||

- | \] | ||

- | |||

- | |||

- | ==== Transitive Closure ==== | ||

- | |||

- | Iterated composition let $r \subseteq A \times A$. | ||

- | \[ | ||

- | \begin{array}{l} | ||

- | r^0 = \Delta_A \\ | ||

- | r^{n+1} = r \circ r^n | ||

- | \end{array} | ||

- | \] | ||

- | So, $r^n$ is n-fold composition of relation with itself. | ||

- | |||

- | Transitive closure: | ||

- | \[ | ||

- | r^* = \bigcup_{n \geq 0} r^n | ||

- | \] | ||

- | |||

- | Equivalent statement: $r^*$ is equal to the least relation $s$ (with respect to $\subseteq$) that satisfies | ||

- | \[ | ||

- | \Delta_A\ \cup\ (s \circ r)\ \subseteq\ s | ||

- | \] | ||

- | or, equivalently, the least relation $s$ (with respect to $\subseteq$) that satisfies | ||

- | \[ | ||

- | \Delta_A\ \cup\ (r \circ s)\ \subseteq\ s | ||

- | \] | ||

- | or, equivalently, the least relation $s$ (with respect to $\subseteq$) that satisfies | ||

- | \[ | ||

- | \Delta_A\ \cup\ r \cup (s \circ s)\ \subseteq\ s | ||

- | \] | ||

- | |||

- | ==== Some Laws in Algebra of Relations ==== | ||

- | |||

- | \[ | ||

- | (r_1 \circ r_2)^{-1} = r_2^{-1} \circ r_1^{-1} | ||

- | \] | ||

- | \[ | ||

- | r_1 \circ (r_2 \cup r_3) = (r_1 \circ r_2) \cup (r_1 \circ r_3) | ||

- | \] | ||

- | \[ | ||

- | (r^{-1})^{*} = (r^{*})^{-1} | ||

- | \] | ||

- | |||

- | Binary relation $r \subseteq A\times A$ can be represented as a directed graph $(A,r)$ with nodes $A$ and edges $r$ | ||

- | * Graphical representation of $r^{-1}$, $r^{*}$, and $(r \cup r^{-1})^{*}$ | ||

- | |||

- | Equivalence relation $r$ is relation with these properties: | ||

- | * reflexive: $\Delta_A \subseteq r$ | ||

- | * symmetric: $r^{-1} \subseteq r$ | ||

- | * transitive: $r \circ r \subseteq r$ | ||

- | |||

- | Equivalence classes are defined by | ||

- | \[ | ||

- | x/r = \{y \mid (x,y) \in r | ||

- | \] | ||

- | The set $\{ x/r \mid x \in A \}$ is a partition: | ||

- | * each set non-empty | ||

- | * sets are disjoint | ||

- | * their union is $A$ | ||

- | Conversely: each collection of sets $P$ that is a partition defines equivalence class by | ||

- | \[ | ||

- | r = \{ (x,y) \mid \exists c \in P. x \in c \land y \in c \} | ||

- | \] | ||

- | |||

- | Congruence: equivalence that agrees with some set of operations. | ||

- | |||

- | Partial orders: | ||

- | * reflexive | ||

- | * antisymmetric: $r \cap r^{-1} \subseteq \Delta_A$ | ||

- | * transitive | ||

- | |||

- | ===== Functions ===== | ||

- | |||

- | **Example:** an example function $f : A \to B$ for $A = \{a,b,c\}$, $B=\{1,2,3\}$ is | ||

- | \[ | ||

- | f = \{ (a,3), (b,2), (c,3) \} | ||

- | \] | ||

- | |||

- | Definition of function, injectivity, surjectivity. | ||

- | |||

- | |||

- | $2^B = \{ A \mid A \subseteq B \}$ | ||

- | |||

- | $(A \to B) = B^A$ - the set of all functions from $A$ to $B$. For $|B|>2$ it is a strictly bigger set than $B$. | ||

- | |||

- | $(A \to B \to C) = (A \to (B \to C))$ (think of exponentiation on numbers) | ||

- | |||

- | Note that $A \to B \to C$ is [[isomorphism|isomorphic]] to $A \times B \to C$, they are two ways of representing functions with two arguments. $(C^B)^A = C^{B \times A}$ | ||

- | |||

- | There is also isomorphism between | ||

- | * n-tuples $(x_1,\ldots,x_n) \in A^n$ and | ||

- | * functions $f : \{1,\ldots,n\} \to A$, where $f = \{(1,x_1),\ldots,(n,x_n) \}$ | ||

- | |||

- | ==== Function update ===== | ||

- | |||

- | Function update operator takes a function $f : A \to B$ and two values $a_0 \in A$, $b_0 \in B$ and creates a new function $f[a_0 \mapsto b_0]$ that behaves like $f$ in all points except at $a_0$, where it has value $b_0$. Formally, | ||

- | \[ | ||

- | f[a_0 \mapsto b_0](x) = \left\{\begin{array}{l} | ||

- | b_0, \mbox{ if } x=a_0 \\ | ||

- | f(x), \mbox{ if } x \neq a_0 | ||

- | \] | ||

- | |||

- | ==== Domain and Range of Relations and Functions ==== | ||

- | |||

- | For relation $r \subseteq A \times B$ we define domain and range of $r$: | ||

- | \[ | ||

- | dom(r) = \{ x.\ \exists y. (x,y) \in r \} | ||

- | \] | ||

- | \[ | ||

- | ran(r) = \{ y.\ \exists x. (x,y) \in r \} | ||

- | \] | ||

- | Clearly, $dom(r) \subseteq A$ and $ran(r) \subseteq B$. | ||

- | |||

- | |||

- | ==== Partial Function ==== | ||

- | |||

- | Notation: $\exists^{\leq 1} x. P(x)$ means $\forall x. \forall y. (P(x) \land P(y))\rightarrow x=y$. | ||

- | |||

- | **Partial function** $f : A \hookrightarrow B$ is relation $f \subseteq A \times B$ such that | ||

- | \[ | ||

- | \forall x \in A. \exists^{\le 1} y.\ (x,y)\in f | ||

- | \] | ||

- | |||

- | Generalization of function update is override of partial functions, $f \oplus g$ | ||

- | |||

- | |||

- | |||

- | |||

- | |||

- | ==== Range, Image, and Composition ==== | ||

- | |||

- | The following properties follow from the definitions: | ||

- | \[ | ||

- | (S \bullet r_1) \bullet r_2 = S \bullet (r_1 \circ r_2) | ||

- | \] | ||

- | \[ | ||

- | S \bullet r = ran(\Delta_S \circ r) | ||

- | \] | ||

- | |||

- | ===== Further references ===== | ||

- | |||

- | * [[sav08:discrete_mathematics_by_rosen|Discrete Mathematics by Rosen]] | ||

- | * [[:Gallier Logic Book]], Chapter 2 | ||