# Mathematical Proof/Logic

As discussed in the introduction, logical statements are different from common English. We will discuss concepts like "or," "and," "if," "only if." (Here I would like to point out that in most mathematical papers it is acceptable to use the term "we" when referring to oneself. This is considered polite by not commanding the audience to do something nor excluding them from the discussion.)

## Truth and statement

editWe encounter *statements* frequently in mathematics.

**Definition.**
(Statement)
A *statement* is a declarative sentence that is either **true** or **false** (but not both), and its *truth value* is **true** (denoted by ) or **false** (denoted by ).

**Example.**
Consider the following sentences:

- The number 9 is even.
- .
- Mathematics is easy.
- How to solve ?
- Please read this book.

- The sentence 1 and 2 above are
*statements*(even if the sentence 1 is false). - The sentences 3, 4 and 5 are
*not*statements, since they are not declarative sentences, and we cannot say whether each of them is true or false.

- In particular, the sentence 3 is an opinion, the sentence 4 is a question, and the sentence 5 is a command. Thus, none of them are declarative.

**Exercise.**

For the sentences that are not statements, there is a special type of sentences among them, namely *open statement*.

**Definition.**
(Open statement)
An *open statement* is a sentence whose truth value depends on the input of certain variable.

**Remark.**

- Since the truth value may change upon input change, we cannot determine the truth value of an open statement (we cannot say it is (always) true or it is (always) false).

**Example.**
Consider the sentence ( is read "such that").

- This sentence is true when , and false otherwise.
- This sentence is an open statement.

To express all possible combinations of *truth values* of some statements, we usually list them in a table, called a *truth table*.

**Example.**
(Truth table for two statements)
For two statements and , the truth table for them is as follows:
and there are four possible combinations of truth values of and .

**Example.**
(Truth table for three statements)
For three statements , and , the truth table for them is as follows:
and there are eight possible combinations of truth values of , and .

**Remark.**

- In general, there are possible combinations of truth values of statements, since each statement has two possible truth values.
- The truth table for three statement may seem to be complicated, and you may miss (or duplicate) some combinations in the truth table if you are not careful enough. Because of this, here is an advice for constructing such truth table:

- for , write four 's and then four 's, from top to bottom;
- for , write in this pattern from top to bottom: ;
- for , write in this pattern from top to bottom: .

## Conjunction, disjunction and negation

editIn this section, we will discuss some ways (conjunction and disjunction) to combine two statements into one statement, which is analogous to Mathematical Proof/Introduction to Set Theory#Set operations, in which two sets are combined into one set. Also, we will discuss a way (negation) to change a statement to another one.

### Conjunctions

edit **Definition.**
(Conjunction)
The *conjunction* of statements and , denoted by , is a statement that is true when *both* and are true, and false otherwise.

**Remark.**

- is read "P and Q", and the conjunction of the statements and can also be expressed by " and " directly.

**Example.**
(Truth table for )
The truth table for (in terms of possible combinations of truth values of and ^{[1]}) is

- We can see that is true only when
*both*and are true, and false otherwise.

### Disjunctions

edit **Definition.**
(Disjunction)
The *disjunction* of statements and , denoted by , is a statement that is false when *both* and are false, and true otherwise.

**Remark.**

- That is, is true when
*at least one*of and is true, and false otherwise. - is read "P or Q", and the disjunction of the statements and can also be expressed by " or " directly.
- The meaning of "or" in mathematics may be different from the meaning of "or" as an English word. For "or" in mathematics, it is used
*inclusively*, i.e. or means or or both. However, as an English word, "or" may be used*exclusively*, i.e. or means or but not both. For example, when someone say "I will go to library or go to park this afternoon.", we interpret this as "I will either go to library, or go to park, but not both.".

**Example.**
(Truth table for )
The truth table for is

- We can see that is false only when
*both*and are false, and true otherwise.

### Negations

edit **Definition.**
The *negation* of a statement , denoted by , is a statement with truth value that is opposite to that of .

**Remark.**

- is read "not P", and the negation of the statement can also be expressed by "not ", or "It is not the case that " directly.
- Another common notation for the negation of a statement is .
- We refer to process of expressing a negation of a statement as
*negating*a statement.

**Example.**
(Truth table for )
The truth table for is

- We can see that always has the opposite truth value to that of .

To negate a statement, we usually add the word "not" into a statement, or delete it from a statement.
For example, the negation of the statement "The number 4 is even." is "The number 4 is *not* even.", and the negation of the statement "The number 1 is not prime." is "The number 1 is ~~not~~ prime."

**Exercise.**
Complete the following truth table:

- We can observe that in some combinations of truth values of and , the truth values of and are different.

**Exercise.**
Let be the statement "The number 3 is even.", and be the statement "The number is irrational."

Write down (in words) the negation of each of and *without using the word "not"*.

- The negation of can be "The number 3 is odd.".
- The negation of can be "The number is rational."

**Exercise.**
Let be the statement and be the statement .

(a) Is the statement true or false?

(b) Is the statement true or false?

(c) Is the statement true or false?

(d) Is the statement true or false?

- First, we can see that both and are false.

(a) Since is false, the answer is *true*.

(b) Since is false, the answer is *true*.

(c),(d) Since and are both true, the answer for (c) and (d) are both *true*.

## Conditionals

editApart from the above ways to form a new statement, we also have another very common way, namely the "if-then combination".
The statement formed using the "if-then combination" is called a *conditional*.

**Definition.**
(Conditionals)
The *conditional* of two statements and is the statement "If then .", denoted by .
and are called the *hypothesis* and *consequent* of the conditional respectively.
The conditional is *false* when is true and is false, and *true* otherwise.

**Example.**
(Truth table for )
The truth table for is

- We can observe that, in particular, even if the hypothesis is false, the conditional is true, by definition
^{[2]}. - Another observation is that when the consequent is true, then no matter the hypothesis is true or false, the conditional must be true (see 1st and 3rd row).

To understand more intuitively why the conditional is always true when the hypothesis is false, consider the following example.

**Example.**
Suppose Bob is taking a mathematics course, called MATH1001. The course instructor of MATH1001 warns Bob that if he fails the final examination of this course, then he will fail this course.
Let be the statement "Bob fails the final examination of MATH1001.", and be the statement "Bob fails MATH1001".
Then, the statement made by the course instructor is " ".

For the statement made by the course instructor to be true, *either* Bob actually fails the final examination of MATH1001, and he actually fails the course, *or* Bob passes the final examination of MATH1001.
In the latter case, whether Bob fails MATH1001 or not does not matter, since the course instructor does not promise anything if Bob passes the final examination of MATH1001 ^{[3]}.

For the statement made by the course instructor to be false, the only situation is that Bob fails the final examination of MATH1001, but he still passes MATH1001 ( is true and is false) (this shows that the instructor is lying!).

### Converse and biconditionals

editAfter discussing *conditional* of and , we will discuss *biconditional* of and . As suggested by its name, there are *two* conditionals involved.
The conditional involved, in addition to , is , which is called the *converse* of the conditional .

In mathematics, given that the conditional is true, we are often interested in knowing whether its *converse* is true as well ^{[4]}.

**Example.**
The converse of the statement made by the course instructor in the previous example is "If Bob fails MATH1001, then he fails the final examination of MATH1001.".

**Definition.**
(Biconditionals)
The *biconditional* of two statements and , denoted by , is , that is, "If then ." and "If then .".

**Remark.**

- We also write
*if and only if*in this case.

- To understand the phrase "if and only if" more clearly, consider the following:
- if and only if can be interpreted as " if and only if ".
- For " if ", it should be easy to accept that it means "If then ".
- For " only if , it means can
*only*be true when is true. That is, is a*necessary condition*for . Thus, we can deduce that when is true, must be true (or else cannot possibly be true). Hence, " only if " means "If then .".

**Example.**
(Truth table for )
The truth table for is

- We can see that is true when both and have the
*same*truth values (i.e. either both and are true, or both and are false), and false otherwise.

### Implication and logical equivalence

editWhen the conditional is *true*, we can say " *implies* ", denoted by .
On the other hand, when does *not* imply , i.e. is *false*, we denote this by .

We have some other equivalently ways to say " implies ", namely

- is a
*sufficient condition*for (or is sufficient for in short)

- We call to be
*sufficient*for since when implies , then " is true" is*sufficient*to deduce " is true".

- We call to be

- is a
*necessary condition*for (or is necessary for in short)

- We call to be
*necessary*for since when implies , the hypothesis cannot be possibly true when the consequent is false (or else the conditional will be false). This explains the*necessity*of " is true".

- We call to be

When , we are often interested knowing whether the *converse* of is true or not, i.e. whether we have or not ^{[5]}.

In the case where but , we have multiple equivalent ways to express this:

- is
*sufficient but not necessary*for .

- From the previous interpretation, when we say is necessary for , we mean . Hence, when is sufficient but not necessary for , we mean and .

- is a
*stronger condition*than (or is*stronger*than in short).

In the case where and as well, the biconditional is also true, and we denote this by . There are also multiple equivalent ways to express this:

- is
*logically equivalent to*.

- We say they are
*logically*equivalent since they always have the same truth values (because is true), which is related to*logic*.

- We say they are

- "
*if and only if*" (is true).

- Recall that we also use " if and only if " to express the
*biconditional*. Thus, technically, we should write " if and only if " is true in the case where . However, we rarely write this in practice, and usually omit the phrase "is true" since it makes the expression more complicated. So, when we write " if and only if " in this context, we mean that the biconditional is true without saying it explicitly. - Usually, when we just write " if and only if ", we have the meaning of the logical equivalence , rather than the statement . If we really want to have the latter meaning, we should specify that " if and only if " refers to a
*statement*.

- Recall that we also use " if and only if " to express the

- is
*necessary and sufficient*for .

- Following the previous interpretation, is necessary and sufficient for means and .

**Theorem.**
(Some laws about conjunction, disjunction and negation)
In the following, , and are arbitrary statements.

- (double negation)
- (commutative law of conjunction)
- (commutative law of disjunction)
- (associative law of conjunction)
- (associative law of conjunction)
- (distributive law)
- (distributive law)
- (De Morgan's law)
- (De Morgan's law)

**Proof.**
They can be shown using truth tables.

**Remark.**

- You may observe that there are also some analogous results in set theory (some even have the same name!), discussed in the chapter about set theory. This is because the results discussed in the chapter about set theory are actually proven from the corresponding results above almost directly.

**Theorem.**
(Bridge between conditional and disjunction)
The statements and are logically equivalent.

**Proof.**
It can be shown using the following truth table:

**Theorem.**
(Logical equivalence of a conditional and its contrapositive)
The *contrapositive* of a conditional is defined to be another conditional , and they are logically equivalent.

**Proof.**
Using the bridge between conditional and disjunction and some other laws, it can be shown as follows:

**Remark.**

- It may seem "surprising" when we "suddenly" negate twice. Indeed, when we are thinking about how to prove the logical equivalence, we do not directly do this from nowhere. Instead, we are "motivated" to do so after observing that and . So, we sort of finish the above series of equivalence "like a sandwich" (from left to right and from right to left simultaneously). This may be useful for other similar proofs.
- This result is very important for Proof by Contrapositive, which will be discussed later.

## Tautologies and contradictions

editBefore defining what tautologies and contradictions are, we need to introduce some terms first.
In the previous sections, we have discussed the meaning of the symbols and .
These symbols are sometimes called *logical connectives*,
and we can use *logically connectives* to make some complicated statements.
Such statements, which are composed of at least one given (or component) statements (usually denoted by etc.) and at least one logical connective, are called *compound statements*.

**Definition.**
(Tautologies and contradictions)
A *compound statement* is called a tautology (contradiction) if it is true (false) for each possible combination of truth values of the component statement(s) that comprise .

**Remark.**

- We use to denote a tautology and to denote a contradiction.
- When a statement is a tautology, we can also write , since it means and always have the same truth value, and because the truth value of is always true, the truth value of is also always true, i.e. is a tautology.

- Because of this, when we want to prove that a statement is
*always true*, one way is to show that . For example, if we want to prove that and are logically equivalent, we may show that .

- Because of this, when we want to prove that a statement is

**Example.**
Let be an arbitrary statement. Then,

- is a tautology.
- is a contradiction.

This is because the truth table for is and the truth table for is

**Example.**
Let and be arbitrary statements.
Then, is a tautology, since its truth table is

**Exercise.**
The following are some further questions on this example.

Prove that is a tautology without using truth tables.

Since the given conditional is a tautology.

A student claims that is a contradiction with the following argument:

- Since , the conditional is a contradiction.

Comment on his claim.

- The claim is wrong.
- The step is incorrect, since we should use distributive law here instead, and we cannot apply associative law with two types of logical connector here.
- Indeed, it is shown in an above question that the conditional is neither tautology nor contradiction.

**Theorem.**
(Laws about tautologies and contradictions)
Let be an arbitrary statement. Then,

- .
- .
- .
- .
- .
- .

**Proof.**
They can be shown using truth tables.

## Quantifiers

editRecall that an *open statement* is a sentence whose truth values depends on the input of certain variable.
In this section, we will discuss a way to change an open statement into a statement, by "restricting the input" using quantifiers, and such statement made is called an quantified statement.
For example, consider the statement "The square of each real number is nonnegative.". It can be rephrased as "For each real number , ."
We can let be the *open statement* " ". Then, it can be further rephrased as "For each real number , ."
In this case, we can observe how an open statement ( ) is converted to a statement (For each real number , ),
and the phrase "for each" is a type of the *universal quantifier*.
Other phrases that are also universal quantifiers include "for every", "for all" and "whenever".
The universal quantifier is usually denoted by (an inverted "A").
After introducing this notation, the statement we mention can be further rephrased as " " (we also use some set notations here).

In general, we can use the universal quantifier to change an open statement to a statement, which is " " in which is a (universal) set (or domain) which restricts the input .

Apart from the universal quantifier, another way to convert an open statement into a statement is using an *existential quantifier*.
Each of the phrases "there exists", "there is", "there is at least one", "for some" ^{[6]}, "for at least one" is referred to as an *existential quantifier*, and is denoted by (an inverted "E"). For example, we can rewrite the statement "The square of some real number is negative." as " " (which is false).

In general, we can use the existential quantifier to change an open statement to a statement, which is " " in which is a (universal) set (or domain) which restricts the input .

Another quantifier that is related to *existential* quantifier is the *unique existential quantifier*. Each of the phrases "there exists a unique", "there is exactly one", "for a unique", "for exactly one" is referred to as *unique existential quantifier*, which is denoted by .
For example, we can rewrite the statement "There exists a unique real number such that ." as " ."
We can express the unique existential quantifier in terms of existential and universal quantifiers, by *defining* " " as
where the left bracket is the existence part, and the right bracket is the uniqueness part.
In words, the expression means

- There exists such that , AND for every , if and , then (i.e., and are actually referring to the same thing, so there is
*exactly one*such that ).

In general, we need to separate the existence and uniqueness part as above to prove statements involving unique existential quantifier.

### Negation of quantified statements

edit **Example.**
The statement " " ^{[7]} can be expressed in words (partially) as "There exists a set such that ."

**Exercise.**
Write down the *negation* of this statement in words (partially).
(Hint: What is the negation of " ( is a nonnegative integer)."? Hence, what is the negation of "There is at least one such that ."?)

- For the hint, the negation of " ( is a nonnegative integer)." is " ", and the negation of "There is at least one such that ." is hence "There is no such that ". It can also be expressed equivalently as "For each , is
*not*satisfied.", or . - Inspired from the hint, the negation of this statement can be written as "For each set , ."

From this example, we can see that the negation of the statement " " is logically equivalent to " ".
Now, it is natural for us to want to know also the negation of the statement " .
Consider this: when it is not the case that is true for each , it means that is false for *at least one* . In other words, .
Hence, we know that the negation of the statement " " is logically equivalent to .

### Quantified statements with more than one quantifier

editA quantified statement may contain more than one quantifier. When only one type of quantifier is used in such quantified statement, the situation is simpler. For example, consider the statement "For each real number and for each real number , is a real number." It can be written as " ". When we interchange " " and " ", the meaning of the statement is not affected (the statement still means "The product of two arbitrary real numbers is a real number.") Because of this, we can simply write the statement as " " without any ambiguity.

For an example that involve two existential quantifiers, consider the statement "There exists an real number and an real number such that is a real number." It can be written as " ." Similarly, interchanging " " and " " does not affect the meaning of the statement (the statement still means "For at least one pair of real numbers and , is a real number.") Because of this, we can simply write the statement as " " without any ambiguity.

However, when both types of quantifier are used in such quantified statement, things get more complicated. For example, consider the statement "For each integer , there exists an integer such that ." This can also be written as " ". It means that for each integer, we can find a (strictly) smaller one, and we can see that this is a true statement. For instance, if you choose , I can choose or . Indeed, for the integer chosen by you, I can always choose my as so that .

Let's see what happen if we interchange " " and " ". The statement becomes
, meaning that there exists an integer such that it is (strictly) smaller than *every* integer ! This is false, since, for example, there is no integer that is (strictly) smaller than itself (which is an integer).
In this example, we can see that interchanging the positions of universal quantifier and existential quantifier can change the meaning of the statement.
Hence, it is very important to understand clearly the meaning of a quantified statement with both universal and existential quantifiers.
To ease the understanding, here is a tips for reading such statement:

- For the variable associated to the quantifier , it
*may*depend on the variable(s) introduced earlier in the statement, but*must*be independent from the variable(s) introduced later in the statement.

What does it mean? For example, consider the above example. *In the first case*, we have . Then, since the variable appears *earlier* than the variable which associated to , *may depend* on (This is similar to the case in English. In a sentence, a thing may depend on other things mentioned earlier.). For instance, when you choose , and I choose . Then, . However, if you change your choice to , then my does not work, and I need to change my to, say, 8 so that .
Then, we can see that depends on in this case.
*In the second case*, we have . In this case, the variable appears *later* than the variable . Hence, the variable must be independent from from . That is, when such is determined, it needs to satisfy for each chosen, and the cannot change depend on what is. Indeed, cannot *possibly* depend on , since is supposed to be determined when is not even introduced!

## Exercises

editIn the following questions, are statements.

A. Construct the truth tables for each the following statements, and also give its converse and contrapositive:

B. Negate the following statements:

- ↑ When we construct a truth table for a statement that involves some statements, we need to also list all possible combinations of truth values of the statements involved, so that we can observe the truth value for that statement under all situations.
- ↑ In this case, we may call this conditional to be
*vacuously true*, since the conditional is meaningless (when the hypothesis is false, no matter the consequent is true or false, the conditional is true. Thus, "the conditional is true" is "useless" when the hypothesis is false). - ↑ For example, it may be the case that Bob does not hand in any assignment, and thus even if he passes the final examination, he still fails the course ( is false and is true). It may also be the case that Bob passes the final examination, and perform consistently well in all assignments, and thus he passes the course ( is false and is false).
- ↑ It is not necessary for to be true when is true. For example, when is false and is true, is (vacuously) true, but is false.
- ↑ It is not necessary for when . See, for example, the third row of the above truth table for , in which is true while is false. Indeed, it is a common mistake to think that we must also have when .
- ↑ "Some" in mathematics always means "at least one".
- ↑ In the form of " ", this statement can also be written as " ."