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Mathematical Preliminaries

CSCI 2670

CSCI 2670 Mathematical Preliminaries

Outline for Today

Material from Chapter 0.

I Sets

I Functions

I Relations

I Graphs

CSCI 2670 Mathematical Preliminaries

Sets

I A set is an unordered collection of objects. The objects in the setare called its elements or members.

I In writing, uppercase Roman letters and braces {...} indicate sets.

S = {1, 2, 3, 5, 8, 13, 21}

I a ∈ S (a 6∈ S) indicates a is (is not) a member of S .

I For sets too big to list, we use “. . .” or state a rule.I S = {2, 4, 6, 8 . . .}I T = {x |x is a positive, even integer}

I Order and repetition do not matter with sets.I {1, 3, 5, 7, 9} and {3, 1, 7, 9, 5} denote the same set.I {a, a, b, b, c , c} and {a, b, c} denote the same set.

CSCI 2670 Mathematical Preliminaries

Common Sets of Numbers

I N = {0, 1, 2, . . .}, the natural numbers.

I Z = {0,±1,±2, . . .}, the integers.

I Q = {pq | p ∈ Z and q ∈ Z, q 6= 0}, the rational numbers.

I R, the real numbers (numbers with infinite decimalrepresentations).

I Some authors use N,Z,Q,R.

I Some use superscripts: Z+ = {1, 2, . . .} (the positive integers).

I Some don’t consider 0 a natural number.

CSCI 2670 Mathematical Preliminaries

Subsets and Supersets

I If all members of A are members of B, then A is a subset of B.I A ⊆ B: A is a subset of B.I A * B: A is not a subset of B.

I If A ⊆ B, then B is a superset of A.

I If A ⊆ B and B ⊆ A, then A = B.

I If A ⊂ B and A 6= B, then A is a proper subset of B.

I Observe that for any set S , S ⊆ S and S 6⊂ S .

CSCI 2670 Mathematical Preliminaries

Subsets and Supersets

I The empty set ∅ (or ∅, or {}) is the set with no members.I For any set S , ∅ ⊆ S .

I Note: {∅} 6= ∅.

I List all of the proper subsets of the following:I {a}I {a, b}I {a, b, c}I {{a, {b}, {c}}I ∅

CSCI 2670 Mathematical Preliminaries

Cardinality

I The cardinality |S | of set S is the number of elements in S .

I For finite sets, |S | will be an integer.

S = ∅ |S | = 0S = {a, a, a} |S | = 1S = {a, b, c} |S | = 3S = {{a}, {b, c}} |S | = 2S = {x | x ∈ N} |S | is “countably infinite”

CSCI 2670 Mathematical Preliminaries

Powersets

I The powerset P(S) of set S is the set of all subsets of S .

I In general, if |S | = n, then |P(S)| = 2n.

S |S | P(S) |P(S)|∅ 0 ? ?{a} 1 ? ?{a, b} 2 ? ?{a, b, c} 3 ? ?{a, b, c , d} 4 ? ?

CSCI 2670 Mathematical Preliminaries

Powersets

I The powerset P(S) of set S is the set of all subsets of S .

I In general, if |S | = n, then |P(S)| = 2n.

S |S | P(S) |P(S)|∅ 0 {∅} 1{a} 1 {∅, {a}} 2{a, b} 2 {∅, {a}, {b}, {a, b}} 4{a, b, c} 3 {∅, {a}, {b}, {c}, {a, b}, {a, c}, {b, c}, {a, b, c}} 8{a, b, c , d} 4 {∅, {a}, {b}, {c}, {d}{a, b}, {a, c}, {a, d}, {b, c},

{b, d}, {c , d}, {a, b, c}, {a, b, d}, {a, c , d}, 16{b, c , d}, {a, b, c , d}}

CSCI 2670 Mathematical Preliminaries

ordered n-tuples

I Let a1, a2, . . . an be elements of a set.

I An ordered list of these elements is a sequence.

I (a1, a2, . . . , an) is called an ordered n-tuple.

I (ai , aj) is an ordered pair.

I The order of the elements matters.I (ai , aj) = (aj , ai ) if and only if ai = aj .I If ai 6= aj , then (ai , aj) 6= (aj , ai ).

I In a sequence or n-tuple, repetitions can occur: (1,1,2,3,5,8).

CSCI 2670 Mathematical Preliminaries

Cartesian Products

I A× B is the Cartesian product (cross product) of sets A and B.

A× B = {(a, b)| a ∈ A and b ∈ B}

I N-ary Cartesian product: If A1, . . ., An are sets,

A1 × . . .× An = {(a1, . . . , an)| ai ∈ Ai for each i = 1, . . . , n}

I An is shorthand for A× A× . . .× A.

I Note that × is not commutative or associative.I A× B 6= B × A.I A× (B × C ) 6= (A× B)× C .

CSCI 2670 Mathematical Preliminaries

Cartesian Products

I Let A = {a, b, c} and B = {0, 1}

I What is A× A, A× B, B × A, and B × B.

I Let R be the set {(1, 1)}. Is R a subset of any of the above?

I Let R be the set {(b, 1), (1, 1)}. Is R a subset of any of the above?

CSCI 2670 Mathematical Preliminaries

Venn Diagrams

I Sets can be represented using Venn diagrams.

I A rectangle represents U, the universal set (the set of everything).

I Sets are drawn using closed shapes inside of U.

I Points inside each shape represent elements of the set.

I (We may cross out a region to indicate that it is empty).

I bob ∈ A, A ⊂ U.

CSCI 2670 Mathematical Preliminaries

Venn Diagrams

I Draw a Venn diagram showing that A ⊆ B.

I Draw a Venn diagram showing that A * B.

I Draw a Venn diagram showing that A * B and B * A.

I Draw a Venn diagram showing that A ⊆ B and A ⊆ C but B 6= C .

CSCI 2670 Mathematical Preliminaries

Set Operations

I Union: A ∪ B = {x |x ∈ A OR x ∈ B}

I Intersection: A ∩ B = {x |x ∈ A AND x ∈ B}I If A ∩ B = ∅, then A and B are disjoint.

I Set Difference: A− B = {x |x ∈ A AND x /∈ B}

I Complement: A = U − A

CSCI 2670 Mathematical Preliminaries

Generalized Union and Intersection

Let A1, A2, . . ., An be sets.

I

n⋃i=1

Ai = A1 ∪ A2 ∪ . . . ∪ An

I

n⋂i=1

Ai = A1 ∩ A2 ∩ . . . ∩ An.

Example: Let A1 = {1, 2, 3}, A2 = {3, 4, 5}, A3 = {1, 3, 6}.

I

3⋃i=0

Ai = {1, 2, 3, 4, 5, 6}

I

3⋂i=0

Ai = {3}

CSCI 2670 Mathematical Preliminaries

Questions

Let U = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, A = {1, 2, 3, 4, 5}, B = {0, 3, 6}.What is:

I A ∪ B

I A ∩ B

I A− B

I B − A

I A

I U

I A ∩ U

CSCI 2670 Mathematical Preliminaries

Functions

I Let A and B be nonempty sets.

I A function f from A to B (f : A→ B) is an assignment of exactlyone element in B to each element in A.

I A is the domain of f .I B is the codomain of f .I {f (a)|a ∈ A} is the range of f .

I f is sometimes called a map, mapping, or transformation.

I If a ∈ A, f (a) denotes the element in B assigned to a.

I If f : A→ B is defined for each a ∈ A, then f is a total function.

I If there is an a ∈ A with f (a) undefined, f is a partial function.

I Functions can be written in prefix notation—g(x , y)—or infixnotation—x + y .

CSCI 2670 Mathematical Preliminaries

Functions

I Given function f : A→ B:I A = {a, b, c , d}.I B = {0, 1, 2, 3}.

a f (x)

a 1b 0c 0d 1

I What’s the domain of f ?

I What’s the codomain of f ?

I What’s the range of f ?

I How would you represent f as a set?

CSCI 2670 Mathematical Preliminaries

Injective, Surjective, and Bijective Functions

Let f : A→ B be a function. f isI injective (one-to-one) if and only if each element of A gets

mapped to a unique element of B.

I surjective (onto) if and only if the codomain of f equals therange of f .

I bijective (a one-to-one correspondence) if it is injective andsurjective.

Important! if f : A→ B is bijective, then |A| = |B|.Is this function bijective? Assume A = {a, b, c , d} and B = {0, 1, 2, 3}.

a f (x)

a 1b 0c 0d 1

CSCI 2670 Mathematical Preliminaries

Which are Bijective?

I Assume f : R→ R.I f (x) = 3x

I f (x) = sin(x) + 20

I f (x) = x2 + 1

I f (x) = x3

I What if f : R+ → R+?

I Suppose g : N→ R. Can g be bijective?

CSCI 2670 Mathematical Preliminaries

Unary, Binary, k-ary Functions

I f : A→ B is a unary function (it has one input).

I f : A1 × A2 → B is a binary function (it has two inputs).I It maps ordered pairs from A1 × A2 to elements of B.

I f : A1 × A2 × . . .× Ak → B is a k-ary (or arity k) function (ithas k inputs).

I It maps ordered k-tuples from A1 × A2 × . . .× Ak to elements of B.

I Finite binary functions can be represented using a table.

I Let A = {1, 2, 3} and B = {2, 5}. Define f : A× B → N as:

f 2 5

1 2 52 4 103 6 15

CSCI 2670 Mathematical Preliminaries

Relations

I A subset of A1 × A2 × . . .× Ak is a k-ary relation.

I Alternatively, a relation can be viewed as a function:

A1 × A2 × . . .× Ak → {TRUE ,FALSE}.

I A function whose codomain is {TRUE ,FALSE} is sometimescalled a predicate (sometimes a property).

I E.g., ≤ is a binary relation on R × R .

I As a set, (4, 4) ∈≤.

I As a function, ≤ (4, 4) = TRUE , but ≤ (5, 4) = FALSE .

CSCI 2670 Mathematical Preliminaries

Equivalence Relations

I Let A be a set.

I An equivalence relation R on A is a relation R ⊆ A× A that is

I reflexive: For each x ∈ A, (x , x) ∈ R.

I symmetric: For each x , y ∈ A, if (x , y) ∈ R, then (y , x) ∈ R.

I transitive: For each x , y , z ∈ A, if (x , y) ∈ R and (y , z) ∈ R, then(x , z) ∈ R.

I “=” is obviously an equivalence relation.

I So is “has the same mother as”.

CSCI 2670 Mathematical Preliminaries

Equivalence Relations

I An equivalence relation R on A is a relation R ⊆ A× A that is

I reflexive: For each x ∈ A, (x , x) ∈ R.

I symmetric: For each x , y ∈ A, if (x , y) ∈ R, then (y , x) ∈ R.

I transitive: For each x , y , z ∈ A, if (x , y) ∈ R and (y , z) ∈ R, then(x , z) ∈ R.

I Consider the following relation: x ≡7 y if and only if x − y is amultiple of 7.

I Is this an equivalence relation?

CSCI 2670 Mathematical Preliminaries

Outline for Today

Material from Chapter 0.

I Graphs

I Recursive Definitions

I Strings and languages

I Logic

I Proofs

For Next Week: Read Ch 1 (Sec 1.1-1.2)

CSCI 2670 Mathematical Preliminaries

Graphs

I A graph G = (V ,E ) consists ofI a nonempty set V of vertices (or nodes) and

I a set E of edges (represented as pairs of vertices from V).

I In an undirected graph, the pairs are unordered—e.g., {u, v}. In adirected graph, the pairs are ordered—e.g., (v , u).

I A loop is an edge that connects one vertex to itself.

I A graph is simple if it there are no duplicate edges between nodes.

I Let G = ({v1, v2, v3, v4}, {{v1, v2}, {v1, v3}, {v1, v4}, {v3, v4}})

CSCI 2670 Mathematical Preliminaries

Subgraphs

I If G1 = (V1,E1) and G2 = (V2,E2) are graphs such that V2 ⊆ V1

and E2 ⊆ E1, then G2 is a subgraph of G1.

I H = ({v1, v3, v4}, {{v1, v3}, {v1, v4}, {v3, v4}}) is a subgraph of G.

CSCI 2670 Mathematical Preliminaries

Degree of Vertices

I In an undirected graph, the degree deg(v) of a vertex v is thenumber of edges incident with it (loops count twice).

I Above, the deg(v2) = 3, and deg(v3) = 2.

CSCI 2670 Mathematical Preliminaries

Degree of Vertices

I In a directed graph, edges are ordered and indicated using arrows.

I The indegree deg−(v) of v is the number of incoming edges.

I The outdegree deg+(v) of v is the number of outgoing edges.

I Above, the deg−(a) = 0 but deg+(a) = 2.

CSCI 2670 Mathematical Preliminaries

Paths in Undirected Graphs

I A path from u to v in an undirected, simple graph is a sequence ofvertices v1, v2, . . . , vn such that u = v1 and v = vn, and for each vi < vn,{vi , vi+1} is an edge. A circuit (cycle) is a path in which u = v .

I A path is simple if no vertex is repeated.

I A circuit is simple if n > 2, v1 = vn, but no other vertex is repeated.

I A graph is connected if there is a path connecting every two vertices.

I The above graph is connected.

CSCI 2670 Mathematical Preliminaries

Trees

I A connected graph without simple cycles is called a tree.

I Each pair of distinct nodes is connected by exactly one simple path.

I A specific node might be designated the root.

I The nodes of degree 1 (other than the root) are leaves.

CSCI 2670 Mathematical Preliminaries

Paths in Directed Graphs

I A path from u to v in a directed graph G = (V ,E ) is a sequence ofvertices v1, v2, . . . , vn such that u = v1 and v = vn, and for each vi < vn,(vi , vi+1) ∈ E .

I G is strongly connected if a directed path connects any two distinctvertices.

I If the underlying undirected graph is connected, G is weakly connected.

I The above graph is not strongly connected.

CSCI 2670 Mathematical Preliminaries

Directed Graphs

Draw the following directed graph G = (V ,E ), where

1. V = {1, 2, 3, 4, 5, 6}, and

2. E = {(1, 2), (1, 5), (2, 1), (2, 4), (5, 4), (5, 6), (6, 1), (6, 3)}

Is it strongly connected?

CSCI 2670 Mathematical Preliminaries

Recursive Definitions

I An infinite set S can be defined recursively.I Basis: A finite number of items is first specified to be in S .

I Recursive Step: A set of operations generates additional membersof S from previously defined members of S .

I A closure condition ensures only objects defined in the basis or viafinite number of applications of the operations are members of S .

CSCI 2670 Mathematical Preliminaries

Recursive Definitions

I A recursive definition of N:I Basis: 0 ∈ N.

I Recursive Step: If n ∈ N, then s(n) ∈ N.

I Only 0 and objects generated by a finite number of applications of sare in N.

I A recursively defined set S can be associated with a sequence ofsets S0, S1, S2 . . ..

I S0: the basis of S .

I Si+1: Si together with items generated by a single application ofoperations in the recursive step.

CSCI 2670 Mathematical Preliminaries

Strings and Languages

I An alphabet is a nonempty, finite set of objects (symbols).I Σ and Γ are usually used to indicate alphabets.

I For instance, Σ1 = {a, b, c , d , e, f }, Σ2 = {0, 1}

I A string over Σ is any finite sequence of symbols from Σ.I 0, 101, and 0101111 are strings over Σ = {0, 1}.I The length |w | of a string w is the number of symbols in w .

I |0| = 1, |101| = 3, and |0101111| = 7.

I We refer to the symbols of w by their index: w = w1w2w3 . . ..

I The empty string ε (or λ) consists of no symbols; |ε| = 0.

I wR = wn . . .w2w1 is the reverse of w = w1w2 . . .wn.

I A consecutive sequence of symbols of w is a substring of w .

CSCI 2670 Mathematical Preliminaries

Strings and Languages

I Let x = x1 . . . xn and y1 . . . ym be strings over some alphabet.

I xy = x1 . . . xny1 . . . ym is the concatenation of x and y .I Σ = {0, 1},I x = 01, y = 001.

I xy = 01001, yx = 00101.

I If w is a string and k ∈ N, then wk is the concatenation of k w ’s.

I If w = 001, then w3 = 001001001.

I Note: w0 = ε for any string w .

CSCI 2670 Mathematical Preliminaries

Σ∗ and Languages

I Let Σ be some alphabet.

I Σ∗ is the set of strings defined as follows:I Basis: ε ∈ Σ∗.

I If w ∈ Σ∗ and a ∈ Σ, then wa ∈ Σ∗.

I Nothing not in the basis or else formed by a finite number ofapplications of the above rule is a member of Σ∗.

I A language L over an alphabet Σ is any subset of Σ∗.

I (In other words, a language is a set of strings).

CSCI 2670 Mathematical Preliminaries

Σ∗ and Languages

I Provide a recursive definition for the language consisting of allstrings representing even binary numbers.

I Provide a recursive definition for the language consisting of allstrings representing odd binary numbers.

I Provide a recursive definition for the language of all bit strings thatread the same forwards and backwards.

CSCI 2670 Mathematical Preliminaries

Boolean (Propositional) Logic

The language of Boolean logic uses the following alphabet.

I Propositional Variables: p, q, r , etc., with/without subscripts.

I The Logical Connectives (operators):Negation ¬ NOTConjunction ∧ ANDDisjunction ∨ ORExclusive Disjunction ⊕ XORImplication → IF...THENBiimplication ↔ IF AND ONLY IF

I Grouping Symbols ‘(’ and ‘)’.

CSCI 2670 Mathematical Preliminaries

Boolean (Propositional) Logic

I Well-formed formulas (wffs) of the logic are defined as follows:I If p is a propositional variable, then p is a wff.

I If p is a wff, then ¬p is a wff.

I If p and q are wffs, then the following are all wffs:

I (p ∧ q), (p ∨ q), (p ⊕ q), (p → q), (p ↔ q)

I No other strings are wffs.

I Note that this is language in the sense described earlier.

I Note also that the wffs are defined recursively.

CSCI 2670 Mathematical Preliminaries

Boolean (Propositional) Logic

I The propositional variables range over {0, 1}.

I The below tables allow values to be assigned to every wff.

p q (p ∧ q) (p ∨ q) (p ⊕ q) (p → q) (p ↔ q)

1 1 1 1 0 1 11 0 0 1 1 0 00 1 0 1 1 1 00 0 0 0 0 1 1

p ¬p1 00 1

A formula p is satisfiable if it is possible to assign 0 and 1 to itspropositional variables such that p evaluates to 1.

CSCI 2670 Mathematical Preliminaries

Proofs: Terminology

I An axiom is a proposition that is taken as true without question.

I A theorem is a proposition that has been proven to be true (it hasbeen shown to deductively follow from definitions and axioms).

I A corollary is a proposition that trivially follows from a theorem.

I A lemma is a minor theorem, usually needed in the proof of moreimportant theorems.

I A conjecture is a proposition that has been claimed to be true butnot proven.

CSCI 2670 Mathematical Preliminaries

Conditional Proofs

I Many mathematical claims are of the form “If P, then Q”.

I One way of proving such a claim is to first assume that P is true,and then show that Q must also be true.

I In the proof, you may only use definitions, axioms, the assumptionof p, as well as any other statements that have already beenproven.

I This is called a conditional proof or direct proof.

CSCI 2670 Mathematical Preliminaries

Proof Strategies: Conditional Proof

Claim: Let A and B be sets. If x ∈ A ∪ B, then x ∈ A ∩ B.

CSCI 2670 Mathematical Preliminaries

Proof Strategies: Conditional Proof

Let A and B be sets. If x ∈ A ∪ B, then x ∈ A ∩ B.

Proof.Suppose x ∈ A ∪ B. By definition of set complements, x /∈ A ∪ B. Bydefinition of A ∪ B, x /∈ A and x /∈ B. Because of this (again using thedefinition of set complements), x ∈ A and x ∈ B. According to thedefinition of set intersection, x ∈ A ∩ B

CSCI 2670 Mathematical Preliminaries

Proofs of Biconditionals/Equality

I To prove “P if and only if Q”, perform two proofs:

I the forward direction (Left-to-Right): “If P, then Q”.

I the reverse direction (Right-to-Left): “If Q, then P”.

I E.g. “x ∈ A ∪ B if and only if x ∈ A ∩ B.”I Show x ∈ A ∪ B implies x ∈ A ∩ B, and

I Show x ∈ A ∩ B implies x ∈ A ∪ B.

I Many proofs for equality are implicitly biconditional proofs.

I E.g., to prove A ∪ B = A ∩ B, you prove the bi-conditional above.

CSCI 2670 Mathematical Preliminaries

Existence proofs: Constructive

I Some mathematical propositions claim the existence of something.

I In a constructive proof, the claim is proven by actually exhibitingor constructing the object.

I Claim: There exists an even prime number.

I Proof: It’s 2.

CSCI 2670 Mathematical Preliminaries

Existence proofs: Nonconstructive

In a nonconstructive proof, the claim is proven, but no object isconstructed or presented.

Claim: There exist irrational numbers x and y such that xy is rational.(Consider a = b =

√2).

CSCI 2670 Mathematical Preliminaries

Existence proofs: Nonconstructive

In a nonconstructive proof, the claim is proven, but no object isconstructed or presented.

Claim: There exist irrational numbers x and y such that xy is rational.(Consider a = b =

√2).

Proof.Let a = b =

√2 (note that

√2 is irrational). ab is either rational or it’s not.

We consider each case.

1. If ab is rational, then the claim is proven.

2. If ab is irrational, then let c = ab =√

2√2. ca = (

√2√2)√2 =

√2√2∗√2

=√

22

= 2. And so ca is rational.

Either way, there exist irrational x and y such that xy is rational.

This is a proof by cases. We haven’t ascertained which case obtains,and so we haven’t actually provided a specific x , y proving the claim.

CSCI 2670 Mathematical Preliminaries

Proof by Contradiction

In a proof by contradiction, to prove P, one shows that ¬P implies acontradiction.

Claim:√

2 is irrational.

CSCI 2670 Mathematical Preliminaries

Proof by Contradiction

In a proof by contradiction, to prove P, one shows that ¬P implies acontradiction.

Claim:√

2 is irrational.

Proof.Suppose for a proof by contradiction that

√2 is rational. Then there

exist integers a and b with b 6= 0 such that√

2 = ab . We can assume

without loss of generality that either a or b is odd, since if they areboth even ( a

b = 2c2d ), we can reduce the fraction further.

Squaring both sides of√

2 = ab yields 2 = a2

b2. And so a2 = 2b2. So a2

is even, which implies that a is even (a = 2k for some integer k). Assuch, a2 = (2k)2 = 4k2 = 2b2, and so 2k2 = b2, which means that b2

and hence b is even. A contradiction! We conclude that√

2 isirrational.

CSCI 2670 Mathematical Preliminaries

Proof by Induction

I Induction may be used to prove each n ∈ N, n > b, has aproperty P.

I Inductive proofs have two parts:I Basis: P(b) is shown to hold.

I Induction: P(k) is assumed to hold (this is the inductivehypothesis), and it is then shown that P(k + 1) must also hold..

I strong induction: In this variant of induction, the inductivehypothesis instead assumes P(j) holds for all j ≤ k .

CSCI 2670 Mathematical Preliminaries

Proof by Induction

Claim: For every integer n > 3, 2n < n!.

CSCI 2670 Mathematical Preliminaries

Proof by Induction

Claim: For every integer n > 3, 2n < n!.

Proof.

I Basis: 24 = 16, and 4! = 4× 3× 2 = 24, and so the claim holdsfor n = 4.

I Induction: Suppose the claim holds for n = k , where k > 2. Then2k < k!. As such, 2(2k) < 2(k!), that is 2k+1 < 2k!. Since 2 < k ,it follows that 2 < k + 1 and so 2k! < (k + 1)k!. From this,2k+1 < (k + 1)!, which is what we wanted to show.

CSCI 2670 Mathematical Preliminaries

Proof by Induction

Claim: Every integer n > 1 is either prime or else equal to a product ofprimes. (Note that if n is not prime, then by definition there existintegers 1 < a, b < n such that n = ab.)

CSCI 2670 Mathematical Preliminaries

Proof by Induction

Claim: Every integer n > 1 is either prime or else equal to a product ofprimes. (Note that if n is not prime, then by definition there existintegers 1 < a, b < n such that n = ab.)

Proof.

I Basis: 2 is prime.

I Induction: Suppose the claim holds for all 2 ≤ n ≤ k and consider k + 1.k + 1 is either prime or not. If k + 1 is prime, then the claim clearlyholds. So suppose that k + 1 is not prime. Then there exist integers1 < a, b < k + 1 such that k + 1 = ab. Since a < k + 1 and b < k + 1,it follows by the inductive hypothesis that each is either prime or elseequal to a product of primes. And so k + 1 is itself equal to a product ofprimes.

Note that this is an example of strong induction. The inductivehypothesis assumes the claim holds for all integers ≤ k and then theproof shows that the claim must also hold for k + 1.

CSCI 2670 Mathematical Preliminaries

Structural Induction (general idea)

I Suppose that P is some property, and we want to show that everywff of propositional logic has property P.

I Basis: We show that every atomic proposition has property P.I Induction: We then show that if p and q have property P (where p

and q are arbitrary), then ¬p, (p ∧ q), (p ∨ q), (p ∨ q), (p → q),and (p ↔ q) have property P.

I This is also a form of induction, one that leverages a recursivedefinition of a set.

I Here, we show that every possible application of a rule for wffsensures that the resulting expression has property P.

CSCI 2670 Mathematical Preliminaries

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