Writing Proofs

The idea of a proof is to show that a statement is definetly and unequivocally true. This is done by providing a logical argument that shows that the statement is true. There are three different types of statements:

Theorem: A statement that is known to be true and therefore can and has been proven.
Conjecture: A statement that is believed to be true or false but has not been proven yet.
Disproof: A statement that is known to be false and therefore can and has been disproven.

When writing a proof we are trying to show that a conjecture is actually a theorem.

Prooving Conditional Statements

We start with the basic building block of proofs, the conditional statement. The conditional statement is a statement of the form "If P then Q", $P \rightarrow Q$ . P is often called the hypothesis and Q is called the conclusion.

So the first step in writing a proof is to write the statement that we want to prove in the form of a conditional statement. This can be done for almost all mathematical statements that we want to prove.

Why do we use conditional statements? Because they are easy to prove if we know that the hypothesis is true then we can show that the conclusion is true thanks to truth table of the conditional statement $P \rightarrow Q$ and the logical inference rule of modus ponens. So in other words we must show that the condition of P being true forces Q to be true.

As a reminder, the truth table of the conditional statement is:

$p$	$q$	$p \implies q$
$0$	$0$	$1$
$0$	$1$	$1$
$1$	$0$	$0$
$1$	$1$	$1$

and the logical inference rule of modus ponens is:

\begin{align*} &p \implies q \\ &p \\ \hline &q \end{align*}

Example

Some examples of conditional statements that we might want to prove are:

If $n$ is an integer and $n$ is even, then $n^2$ is even.
If $n$ is an integer and $n$ is odd, then $n^2$ is odd.

Direct Proof

Direct proof is the most common type of proof. It is a proof that starts with the hypothesis and then uses logical inference rules and known mathematical facts, properties, rules and theorems to show that the conclusion is true.

So the setup is pretty simple, we start with the hypothesis and then we write "Proof" to indicate that we are starting the proof. Then we write the conclusion starting with "therefore" or "thus" or "so" to indicate that we are showing that the conclusion is true. To show that the proof is then completed after the conclusion we often write "QED" which stands for "Quod Erat Demonstrandum" which is Latin for "that which was to be demonstrated" or we draw a square at the end of the proof $\blacksquare$ . Inbeween we write the logical steps that show that the conclusion is true.

Proof

If P, then Q

Proof: Suppose P is true..

Some logical steps...

Therefore Q is true... $\blacksquare$

Example

If $n$ is an integer and $n$ is odd, then $n^2$ is odd.

Proof: Suppose $n$ is an integer and $n$ is odd.

Then $n = 2k + 1$ for some integer $k$ , by the definition of an odd integer.
Thus $n^2 = (2k + 1)^2 = 4k^2 + 4k + 1 = 2(2k^2 + 2k) + 1$ .
So $n^2 = 2m + 1$ for some integer $m$ and where $m = 2k^2 + 2k$ .
Thus $n^2=2m+1$ for some integer $m$ .

Therefore $n^2$ is odd, by the definition of an odd integer. $\blacksquare$

Proof by Cases

When proving a statement is true, we sometimes have to examine multiple cases to show that the statement is true for all possible scenarios. By splitting up the proof into multiple cases it can make finding the proof easier but also easier to understand. However, when using proof by cases we must make sure that the cases are exhaustive. That is, we must show that all possible cases are covered.

Proof

If P, then Q

Proof: Suppose P is true..

Then we can consider the following cases:

Case 1: Suppose something.

Then some logical steps...

Case 2: Suppose something else.

Then some logical steps...

These cases show that Q is true... $\blacksquare$

Example

If $n$ is an integer then $1 + (-1)^n (2n-1)$ is a multiple of 4.

Proof: Suppose $n$ is an integer.

Then we can consider the two cases where $n$ is even or $n$ is odd. Let's consider these two cases separately:

Case 1: Suppose $n$ is even.

Then $n = 2k$ for some integer $k$ and $(-1)^n = 1$ .
Thus $1 + (-1)^n (2n-1) = 1 + 1(2(2k)-1) = 1 + 1(4k-1) = 1 + 4k - 1 = 4k$ , which is a multiple of 4.

Case 2: Suppose $n$ is odd.

Then $n = 2k + 1$ for some integer $k$ and $(-1)^n = -1$ .
Thus $1 + (-1)^n (2n-1) = 1 + (-1)(2(2k+1)-1) = 1 + (-1)(4k+2-1) = 1 + (-1)(4k+1) = 1 - 4k - 1 = -4k$ , which is a multiple of 4.

These cases show that $1 + (-1)^n (2n-1)$ is always a multiple of 4. $\blacksquare$

Sometimes two or more cases are almost the same and it can be annyoing to write them out separately. In these cases we can say that the other case is the same as the first case and then we can combine the cases into one case. This is called "without loss of generality" or WLOG.

Example

If two integers $a$ and $b$ have opposite parity, then their sum $a+b$ is odd.

Proof: Suppose $a$ and $b$ are integers with opposite parity.

Without loss of generality, let's consider the case where $a$ is even and $b$ is odd.

Then $a = 2k$ and $b = 2m + 1$ for some integers $k$ and $m$ .
Thus $a + b = 2k + 2m + 1 = 2(k + m) + 1$ , which is odd.

Therefore, the sum of two integers with opposite parity is odd. $\blacksquare$

Contrapositive Proof

Contrapositive proofs work just like direct proofs by using logical inference rules and known mathematical facts, properties, rules and theorems to show that the conclusion is true. The only difference is that we prove the contrapositive of the original statement. So rather than proving the statement "If P then Q", we prove the statement "If not Q then not P". We do this because in some cases it is easier to prove the contrapositive statement than the original statement. However, we must be careful when creating the contrapositive statement to make sure that it is equivalent to the original statement as in some cases you might need to use De Morgan's Laws to show that the contrapositive statement is equivalent to the original statement. As a reminder, here are De Morgan's Laws:

\begin{align*} \lnot (p \land q) &\equiv \lnot p \lor \lnot q \\ \lnot (p \lor q) &\equiv \lnot p \land \lnot q \end{align*}

If we compare the truth table of the conditional statement and the contrapositive statement we can see that they are equivalent. Which is what allows us to use the contrapositive statement to prove the original statement.

$p$	$q$	$\lnot p$	$\lnot q$	$p \implies q$	$\lnot q \implies \lnot p$
$0$	$0$	$1$	$1$	$1$	$1$
$0$	$1$	$1$	$0$	$1$	$1$
$1$	$0$	$0$	$1$	$0$	$0$
$1$	$1$	$0$	$0$	$1$	$1$

The setup for a contrapositive proof is then slightly different from a direct proof.

Proof

If P, then Q

Proof: Suppose P is not true..

Some logical steps...

Therefore Q is not true... $\blacksquare$

Example

Lets compare the direct proof and the contrapositive proof of the statement "If $n$ is an integer and $7n + 9$ is even, then $n$ is odd."

Direct Proof:

Proof: Suppose $n$ is an integer and $7n + 9$ is even.

Then $7n + 9 = 2k$ for some integer $k$ .
Subtracting $6n + 9$ from both sides gives $n = 2k - 6n - 9$ .
Thus $n = 2k -6n - 9 = 2k -6n -10 + 1 = 2(k - 3n - 5) + 1$ .
Consequently, $n = 2m + 1$ , where $m = k - 3n - 5$ .

Therefore, $n$ is odd by the definition of an odd integer. $\blacksquare$

Contrapositive Proof:

Proof: Suppose $n$ is not odd.

Then $n$ is even, so $n = 2k$ for some integer $k$ .
Then $7n + 9 = 7(2k) + 9 = 14k + 9 = 2(7k + 4) + 1$ .
Consequently, $7n + 9 = 2m + 1$ , where $m = 7k + 4$ .

Therefore, $7n + 9$ is odd by the definition of an odd integer and therefore not even. $\blacksquare$

Proof by Contradiction

Unlike direct and contrapositive proofs, proof by contradiction can be used to prove any statement, not just conditional statements. The idea behind proof by contradiction is to assume that the statement we want to prove is false and then show that this assumption leads to non-sense or a contradiction. This contradiction then shows that the original statement must therefore be true. So for the statement $P$ we assume $\lnot P$ and performing some logical steps and arrive to a contradiction of the form $C \land \lnot C$ .

Proof

Proof: Suppose for the sake of contradiction that $\lnot P$ is true.

Some logical steps...

This leads to the contradiction $C \land \lnot C$ . Therefore $P$ is true. $\blacksquare$

So in other words we prove that $\lnot P$ being true forces a contradiction so $\lnot P \implies C \land \lnot C$ . To see that this is the same as prooving $P$ is true let's look at the truth tables of the different statements:

$P$	$C$	$\lnot P$	$C \land \lnot C$	$\lnot P \implies C \land \lnot C$
$1$	$1$	$0$	$0$	$1$
$1$	$0$	$0$	$0$	$1$
$0$	$1$	$1$	$0$	$0$
$0$	$0$	$1$	$0$	$0$

Example

The square root of 2 is irrational.

Proof: Suppose for the sake of contradiction that the square root of 2 is not irrational, so it is rational.

Then $\sqrt{2} = \frac{a}{b}$ for some integers $a$ and $b$ and let's assume that $a$ and $b$ have no common factors, so $\frac{a}{b}$ is fully reduced.
This means that $a$ and $b$ are not both even, so at least one of them is odd. Because if both were even then they would have a common factor of 2.
Squaring both sides gives $2 = \frac{a^2}{b^2}$ , so $a^2 = 2b^2$ .
This means that $a^2$ is even which implies that $a$ is even. Thus as we stated before $a$ and $b$ are not both even, so $b$ is odd.
Since $a$ is even, $a = 2k$ for some integer $k$ . Plugging this into $a^2 = 2b^2$ gives $4k^2 = 2b^2$ or $2k^2 = b^2$ .
This means that $b^2$ is even which implies that $b$ is even. But we stated before that $b$ is odd, so we have a contradiction.

This leads to the contradiction that $b$ is both odd and even. Therefore the square root of 2 is irrational. $\blacksquare$

Proof by Contradiction of Conditional Statements

Using proof by contradiction we can prove simple statements as seen above but we can also use it to prove conditional statements. To prove a conditional statement by contradiction we assume that the statement is false or in other words that $\lnot (P \implies Q)$ is true. However, from the truth table of the conditional statement we know that $P \implies Q$ being false means that $P$ can still be true and $Q$ can be false. So we assume that $P$ is true and $Q$ is false and then we show that this assumption leads to a contradiction.

Proof

If P, then Q

Proof: Suppose for the sake of contradiction that $P$ is true and $Q$ is false.

Some logical steps...

This leads to the contradiction $C \land \lnot C$ . Therefore $P \implies Q$ is true. $\blacksquare$

Example

If $n$ is an integer and $n^2$ is even, then $n$ is even.

Proof: Suppose for the sake of contradiction that $n$ is an integer and $n^2$ is even, then $n$ is not even so $n$ is odd.

Then $n = 2k + 1$ for some integer $k$ .
Thus $n^2 = (2k + 1)^2 = 4k^2 + 4k + 1 = 2(2k^2 + 2k) + 1$ .
So $n^2 = 2m + 1$ for some integer $m$ and where $m = 2k^2 + 2k$ .
Consequently, $n^2 = 2m + 1$ for some integer $m$ , so $n^2$ is odd by the definition of an odd integer.

This leads to the contradiction that $n^2$ is both even and odd. Therefore if $n$ is an integer and $n^2$ is even, then $n$ is even. $\blacksquare$

Prooving Biconditional Statements

Some statements can not be put into the form of a conditional statement but rather a biconditional statement. The biconditional statement is a statement of the form "P if and only if Q", $P \iff Q$ . However, we know that $P \iff Q$ is equivalent to $(P \implies Q) \land (Q \implies P)$ , so we can prove a biconditional statement by proving the two conditional statements that make up the biconditional statement.

Proof

P if and only if Q

Proof:

If P, then Q: Suppose P is true..

Some logical steps...

Conversely, if Q, then P: Suppose Q is true..

Some logical steps...

Therefore P if and only if Q. $\blacksquare$

Example

An integer $n$ is odd if and only if $n^2$ is odd.

Proof:

If $n$ is odd, then $n^2$ is odd.: Suppose $n$ is odd.

Then $n = 2k + 1$ for some integer $k$ .
Thus $n^2 = (2k + 1)^2 = 4k^2 + 4k + 1 = 2(2k^2 + 2k) + 1$ .
So $n^2 = 2m + 1$ for some integer $m$ and where $m = 2k^2 + 2k$ .
Consequently, $n^2 = 2m + 1$ for some integer $m$ , so $n^2$ is odd by the definition of an odd integer.

Conversely, if $n^2$ is odd, then $n$ is odd.: Suppose for the sake of contradiction that $n^2$ is odd and $n$ is not odd.

Then $n$ is even, so $n = 2k$ for some integer $k$ .
Then $n^2 = (2k)^2 = 4k^2 = 2(2k^2)$ .
So $n^2 = 2m$ for some integer $m$ and where $m = 2k^2$ .
Consequently, $n^2 = 2m$ for some integer $m$ , so $n^2$ is even by the definition of an even integer.
This leads to the contradiction that $n^2$ is both odd and even. Therefore if $n^2$ is odd, then $n$ is odd.

Therefore we've shown that an integer $n$ is odd if and only if $n^2$ is odd. $\blacksquare$

Universal and Existential Quantifiers

When we are proving conditional statements we are actually proving statements of the form $\forall x, P(x) \implies Q(x)$ . So by default we are actually already proving universally quantified statements. However, sometimes we want to prove an existential statement of the form $\exists x, P(x)$ . To prove an existential statement we must show that there exists at least one element that satisfies the statement. This can be done by providing an example of an element that satisfies the statement.

Example

There exists an even prime number.

Proof: The number 2 is an even prime number.

However, often a single example is not enough to prove a conditional statement but rather we need to show that an example does exist. This is the difference between a constructive proof and a non-constructive proof. In a constructive proof we provide a specific example that satisfies the statement, while in a non-constructive proof we show that an example does exist but we don't provide a specific example.

Below is an example of a non-constructive proof for a conditional statement containing an existential statement.

Example

If $a,b \in \mathbb{N}$ , then there exist integers $k$ and $l$ for which $gcd(a,b) = ak + bl$ , where $gcd(a,b)$ is the greatest common divisor of $a$ and $b$ .

This is a conditional statement with an existential statment embedded in it:

a,b \in \mathbb{N} \implies \exists k,l \in \mathbb{Z}, gcd(a,b) = ak + bl

Proof: Suppose $a,b \in \mathbb{N}$ .

Then the set $S = \{ak + bl \mid k,l \in \mathbb{Z}\}$ is a set of all possible linear combinations of $a$ and $b$ .
Let $d$ be the smallest positive integer in $S$ .
Then $d = ak + bl$ for some integers $k$ and $l$ .
To show that $d$ is the greatest common divisor of $a$ and $b$ we must show that $d$ divides both $a$ and $b$ .
Let $r$ be the remainder when $a$ is divided by $d$ , so $a = dq + r$ for some integers $q$ and $r$ where $0 \leq r < d$ .
Then $r = a - dq = a - q(ak + bl) = a(1 - qk) + b(-ql)$ .
Therefore $r$ has the form $ak + bl$ and is in the set $S$ . However, $r < d$ so $r = 0$ . And thus $d$ divides $a$ .
Similarly, $d$ divides $b$ .
Now we must show that $d$ is the greatest common divisor of $a$ and $b$ .
We know that $a = gcd(a,b)m$ and $b = gcd(a,b)n$ for some integers $m$ and $n$ .
Then $d = ak + bl = gcd(a,b)km + gcd(a,b)ln = gcd(a,b)(km + ln)$ .
Thus $d$ is a multiple of $gcd(a,b)$ and therefore $d \geq gcd(a,b)$ .
However, $d$ can't be greater than $gcd(a,b)$ because $d$ is the smallest positive integer in $S$ .

Therefore, there exist integers $k$ and $l$ for which $gcd(a,b) = ak + bl$ . $\blacksquare$

Uniqueness Proofs

Sometimes we want to show that there is only one element that satisfies a statement. In other words an existenial statement of the form "there exists a unique element $x$ such that $P(x)$ ". So there is only exactly one element that satisfies the statement. To prove that there is only one element that satisfies the statement we must produce an element that satisfies the statement and then show that no other element satisfies the statement.

Example

If $a,b \in \mathbb{R} \land a \neq 0$ , then there exists a unique real number $x$ such that $ax + b = 0$ .

Proof: Suppose $a,b \in \mathbb{R}$ and $a \neq 0$ .

Then $x = -\frac{b}{a}$ is a real number that satisfies $ax + b = 0$ .
To show that $x$ is unique we must show that no other real number satisfies $ax + b = 0$ .
Suppose $s$ is another real number that satisfies $as + b = 0$ .
Then $as + b = ax + b$ so $as = ax$ and thus $s = x$ .

Therefore there exists a unique real number $x$ such that $ax + b = 0$ . $\blacksquare$

Disproving Statements

So far we have only looked at how to prove that a statement is true. However, sometimes we want to show that a statement is false. Just because we can't find a proof that a statement is true doesn't mean that the statement is false. So we want a way to show that a statement is always false. This is called a disproof, and the process of showing that a statement is false is called disproving.

The process of disproving is actually pretty simple, rather than proving that the statement is true we prove that the statement is false. This is done by showing that the negation of the statement is true. So if the statement is $P$ we show that $\lnot P$ is true.

Disproof of Universal Statements

We already know that by default a statement is actually a universally quantified statement of the form:

\forall x, P(x)

So to disprove a universal statement we must show that there exists at least one element that does not satisfy the statement, in other words:

\lnot \forall x, P(x) \equiv \exists x, \lnot P(x)

This is called finding a counterexample to the statement. A counterexample is an element that does not satisfy the statement. Just providing a single counterexample isn't great, you should also show how the counterexample was found and why it is a counterexample.

Example

For all integers $n$ , $f(n) = n^2 -n + 11$ is a prime number.

| $n$ | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | | $f(n)$ | 11 | 11 | 13 | 17 | 23 | 31 | 41 | 53 | 67 | 83 | 101 | 121 | 143 |

For $n=11$ , $f(11) = 11^2 - 11 + 11 = 11^2$ which is not a prime number. Therefore the statement is false.

Disproof: The statement "For all integers $n$ , $f(n) = n^2 -n + 11$ is a prime number" is false. For a counterexample we can use $n=11$ where $f(11) = 11^2 - 11 + 11 = 11^2$ which is not a prime number. $\blacksquare$

The same goes for a conditional statement, to disprove a conditional statement $P \implies Q$ we must show that there exists an element for which $P$ is true but $Q$ is false as this makes the statement false.

Example

If $A, B$ and $C$ are sets, then $A \setminus (B \cap C) = (A \setminus B) \cap (A \setminus C)$ .

To see where the counterexample comes from we can draw two venn diagrams, one for the left hand side and one for the right hand side.

Disproof: The statement "If $A, B$ and $C$ are sets, then $A \setminus (B \cap C) = (A \setminus B) \cap (A \setminus C)$ " is false. For a counterexample we can use $A = \{1,2,3\}$ , $B = \{1,2\}$ and $C = \{2,3\}$ . Then $A \setminus (B \cap C) = \{1,3\}$ and $(A \setminus B) \cap (A \setminus C) = \{\}$ . Therefore $A \setminus (B \cap C) \neq (A \setminus B) \cap (A \setminus C)$ . $\blacksquare$

Disproof of Existential Statements

If we want to disprove an existential statement of the form:

\exists x, P(x)

We must proove the negation of the statement:

\lnot \exists x, P(x) \equiv \forall x, \lnot P(x)

So in other words we must show that for all elements the statement is false. However this is again equivalent to just proving a universal statement. So to disprove an existential statement we do not use a counterexample but rather a proof that the statement is false for all elements.

Example

There exists a real number $x$ such that $x^4 < x < x^2$ .

Disproof: Suppose for the sake of contradiction that there exists a real number $x$ such that $x^4 < x < x^2$ .

Then $x$ must be positive as it is greater than the non-negative number $x^4$ .
We can then devide the inequality by $x$ to get $x^3 < 1 < x$ .
And subtract 1 from all sides to get $x^3 - 1 < 0 < x - 1$ .
We can then extract the factors of the left hand side to get $(x-1)(x^2 + x + 1) < 0 < x - 1$ and devide by $x-1$ to get $x^2 + x + 1 < 0 < 1$ .
However the left hand side is always positive so we have a contradiction.

This leads to the contradiction that $x^4$ is both positve and negative. Therefore there exists no real number $x$ such that $x^4 < x < x^2$ . $\blacksquare$

Info

Notice that we used a proof by contradiction to disprove the statement. Because we want to disprove $P$ we show that $\lnot P$ is true. Then to prove that $\lnot P$ is true by contradiction we assume that $\lnot \lnot P$ is true which is equivalent to $P$ being true and then we show that this assumption leads to a contradiction. This is the same as proving that $P$ is false.

Proof by Induction

We have all been introduced to one of the problems that gauss solved as a child. The problem was to find the sum of the first $n$ natural numbers. Gauss solved this problem by noticing that the sum of the first $n$ natural numbers can be written as follows:

\sum_{i=1}^{n} i = 1 + 2 + 3 + \ldots + n = \frac{n(n+1)}{2}

The question now is how could we actually prove this? I visual proof would be to write out the first $n$ natural numbers in a row and then write them out in reverse order below. Then we can see that the sum of the first and last number is $n+1$ , the sum of the second and second to last number is $n+1$ and so on.


$1$	$2$	$3$	$\ldots$	$n-2$	$n-1$	$n$
$n$	$n-1$	$n-2$	$\ldots$	$3$	$2$	$1$
$n + 1$	$n + 1$	$n + 1$	$\ldots$	$n + 1$	$n + 1$	$n + 1$

So the result of the first $n$ natural numbers is $n(n+1)$ . However, we can see that we have counted each number twice so we must divide by 2 to get the result $\frac{n(n+1)}{2}$ . The problem with this proof is that it is not a proof but rather an explanation of why the formula is true, we want to prove that the formula is true for all $n$ .

This type of proof is often used in computer sciene where we want to prove that an algorithm has a certain time complexity. In those proofs we often have some recursive relation and using telescoping sums we can find a closed form solution to the sum. We can then prove that the closed form solution is the same as the recursive relation by using proof by induction.

This is where proof by induction comes in. We want to prove the statement "For all $n \in \mathbb{N}$ the sum of the first $n$ natural numbers is $\frac{n(n+1)}{2}$ ". We notice that this statement is a universally quantified statement and could therefore also be written as a list of statements:

\begin{align*} 1 &= \frac{1(1+1)}{2} \\ 1 + 2 &= \frac{2(2+1)}{2} \\ 1 + 2 + 3 &= \frac{3(3+1)}{2} \\ &\vdots \\ \sum_{i=1}^{n} i &= \frac{n(n+1)}{2} \end{align*}

Proof by induction is exactly for this case where we have an infinite number of statements that we want to prove. The structure of a proof by induction is as follows:

Base Case: Prove that the statement is true for the first element, $n=1$ .
Inductive Hypothesis: Assume that the statement is true for some $n=k$ .
Inductive Step: Show that if the statement is true for $n=k$ then it is also true for $n=k+1$ . Notice that this is an implication, so we are proving a conditional statement.

The analogy for why this works can be seen as a row of dominos. If we can show that the first domino falls (is true) and show that if the $k$ 'th domino falling causes the $k+1$ 'th domino to fall then all dominos must fall.

A visual representation of the principle of mathematical induction.

Proof

The statements $S_1, S_2, S_3, \ldots$ are all true.

Proof:

Base Case: Show that $S_1$ is true.
Inductive Hypothesis: Assume that $S_k$ is true for some $k$ .
Inductive Step: Show that if $S_k$ is true then $S_{k+1}$ is true, so $S_k \implies S_{k+1}$ .

Therefore, the statements $S_1, S_2, S_3, \ldots$ are all true. $\blacksquare$

Because of the inductive hypothesis we know that the statement is true for $n=k$ so the left hand side of the implication is true. Then we must show that the statement is true for $n=k+1$ so the right hand side of the implication is true. This often involves some algebraic manipulation using the inductive hypothesis to show that the statement is true for $n=k+1$ .

Example

For all $n \in \mathbb{N}$ , the sum of the first $n$ natural numbers is $\frac{n(n+1)}{2}$ .

Proof:

Base Case: For $n=1$ , the sum of the first natural number is $1 = \frac{1(1+1)}{2} = \frac{2}{2} = 1$ .
Inductive Hypothesis: Assume that the sum of the first $k$ natural numbers is $\frac{k(k+1)}{2}$ .
Inductive Step: Show that if the sum of the first $k$ natural numbers is $\frac{k(k+1)}{2}$ then the sum of the first $k+1$ natural numbers is $\frac{(k+1)(k+2)}{2}$ .

Because of the inductive hypothesis we know the sum of the first $k$ natural numbers is $\frac{k(k+1)}{2}$ .
So we must show that the sum of the first $k+1$ natural numbers is $\frac{(k+1)(k+2)}{2}$ .
The sum of the first $k+1$ natural numbers is $1 + 2 + 3 + \ldots + k + (k+1)$ .
By the inductive hypothesis we know that the sum of the first $k$ natural numbers is $\frac{k(k+1)}{2}$ .
So the sum of the first $k+1$ natural numbers is $\frac{k(k+1)}{2} + (k+1) = \frac{k(k+1) + 2(k+1)}{2} = \frac{(k+1)(k+2)}{2}$ .

Therefore, for all $n \in \mathbb{N}$ , the sum of the first $n$ natural numbers is $\frac{n(n+1)}{2}$ . $\blacksquare$

Strong Induction

hard to show that S_k implies S_k+1 but can show that some lower S_m implies S_k+1 for m < k

Assume that all S_k are true for k < n and show that this forces S_k+1 to be true.

Why do I need multiple base cases?

Smallest Counterexample

hybrid between induction and contradiction

Proofs Involving Sets

Combinatorial Proofs

??? annyoing

Resolution Proofs

????

Combinatorics Functions

$p$	$q$	$\lnot p$	$\lnot q$	$p \implies q$	$\lnot q \implies \lnot p$
$0$	$0$	$1$	$1$	$1$	$1$
$0$	$1$	$1$	$0$	$1$	$1$
$1$	$0$	$0$	$1$	$0$	$0$
$1$	$1$	$0$	$0$	$1$	$1$

$P$	$C$	$\lnot P$	$C \land \lnot C$	$\lnot P \implies C \land \lnot C$
$1$	$1$	$0$	$0$	$1$
$1$	$0$	$0$	$0$	$1$
$0$	$1$	$1$	$0$	$0$
$0$	$0$	$1$	$0$	$0$

$p$	$q$	$\lnot p$	$\lnot q$	$p \implies q$	$\lnot q \implies \lnot p$
$0$	$0$	$1$	$1$	$1$	$1$
$0$	$1$	$1$	$0$	$1$	$1$
$1$	$0$	$0$	$1$	$0$	$0$
$1$	$1$	$0$	$0$	$1$	$1$

$P$	$C$	$\lnot P$	$C \land \lnot C$	$\lnot P \implies C \land \lnot C$
$1$	$1$	$0$	$0$	$1$
$1$	$0$	$0$	$0$	$1$
$0$	$1$	$1$	$0$	$0$
$0$	$0$	$1$	$0$	$0$

$p$	$q$	$\lnot p$	$\lnot q$	$p \implies q$	$\lnot q \implies \lnot p$
$0$	$0$	$1$	$1$	$1$	$1$
$0$	$1$	$1$	$0$	$1$	$1$
$1$	$0$	$0$	$1$	$0$	$0$
$1$	$1$	$0$	$0$	$1$	$1$

$P$	$C$	$\lnot P$	$C \land \lnot C$	$\lnot P \implies C \land \lnot C$
$1$	$1$	$0$	$0$	$1$
$1$	$0$	$0$	$0$	$1$
$0$	$1$	$1$	$0$	$0$
$0$	$0$	$1$	$0$	$0$