13 - Disquality

In addition to definitions, like Odd and Even, Mathlib also contains lemmas and theorems we can use.

We'll see how to use a lemma to support a simple disequality proof.

Task

Given a natural number $n<5$, show that

$$n \neq 5$$

The expression $n \ne 5$ is a disequality, whereas the hypothesis n<5 is an inequality.

Maths

It is common knowledge that, given two natural numbers $a$ and $b$, if $a<b$ is true, then we can say $a\ne b$. 

That common knowledge might seem too trivial to fuss over, but we'll think of it as a small lemma.

If we can show $n<5$, then we can conclude $n \ne 5$, using that lemma.

Let's write a proof that uses this lemma.

$$\begin{align}n&<5&&\text{hypothesis}\tag{1}\label{13.1}\\n&\neq5&&\text{proof objective }\tag{2}\label{13.2}\\&&&\\a<b&\implies a\neq b&&\text{existing lemma}\tag{3}\label{13.3}\\&&&\\n&<5&&\text{sufficient goal, by lemma }(\ref{13.3})\tag{4}\label{13.4}\\&&&\\n&<5&&\text{using }(\ref{13.1})\tag{5}\label{13.5}\\&&&\\n<5&\implies n\neq5&&\text{by lemma }(\ref{13.3})\tag*{\(\Box\)}\end{align}$$

This may look a little over-cooked, but the small steps will help us write a Lean proof. 

We start with the hypothesis $n<5$, and our proof objective $n\ne5$. 

We know about a lemma (3) applicable to natural numbers, that if $a<b$ then $a\ne b$. So if we can prove $n<5$ then we can conclude $n\ne5$. 

This changes our proof goal from $n\ne5$ to $n<5$. 

To prove $n<5$ is easy because we're given it by hypothesis (1]). So $n<5$, and by lemma (3) we finally conclude $n\ne5$.

Code

The following Lean program proves that a natural number $n \ne 5$, given $n < 5$.

-- 13 - Lemma: Not Equal from Less Than

import Mathlib.Tactic

example {n : ℕ} (h: n < 5): n ≠ 5 := by
  apply ne_of_lt
  exact h

The proof header declares n as a natural number, establishes the hypothesis h: n < 5, and specifies the proof objective n ≠ 5.

The apply instruction applies a lemma or theorem to the current goal, usually resulting in a change in goal.

Here, it applies a lemma named ne_of_lt, which means “not equal from less than”. It allows us to prove the current “not equal” goal by instead proving a “less than” goal.

The Infoview will show that apply ne_of_lt does indeed change the current proof goal from n ≠ 5 to n < 5.

The current goal is now n < 5. We could use apply h to resolve the goal, but since the goal matches exactly the hypothesis h, we can instead use the instruction exact h. 

Notice that exact is applying a hypothesis here, not a lemma from Mathlib. The difference is not significant because both hypotheses and lemmas state facts.

We could have used a calc section to prove n < 5 but in this case a multi-line calc section, although familiar, is not warranted.

It may be helpful to correlate this new code back to the maths proof. Here apply ne_of_lt corresponds to line (4) of the maths proof, and exact h corresponds to line (5).

Apply & Exact

We can use apply wherever we use exact. The benefit of exact is that it is stricter than apply. 

The hypothesis or lemma must exactly match the current goal, and if a misunderstanding has led to that not being true, it will be exposed immediately.

Infoview

Placing the cursor before apply ne_of_lt shows the original proof goal.

n : ℕ
h : n < 5
⊢ n ≠ 5

Moving the cursor to the beginning of the next line after apply ne_of_lt shows the goal has indeed changed.

n : ℕ
h : n < 5
⊢ n < 5

Lemmas & Theorems

The distinction between what is called a lemma or a theorem in Mathlib is not precise. Ultimately it doesn't matter as both are used in the same way.

Searching for suitable lemmas and theorems in Mathlib is currently not ideal. Many do conform to a naming convention, which helps. The exercise at the end provides a opportunity to practice finding a lemma using the naming convention.

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Easy Exercise

Write a Lean program to prove $n\ne5$, given $n>5$, where $n$ is a natural number.

The proof will be almost exactly the same as this chapter's example, except the lemma will be “not equal from greater than”. 

Work out the required lemma's Mathlib name fom the naming convention, or search the online Lean documentation to find it.

12 - Odd & Even

In Part III we'll practise using lemmas and definitions in our proofs. 

There is a huge body of commonly agreed knowledge that mathematicians refer to in their own proofs. A large, and ever-growing, number of these lemmas, theorems, and definitions are encoded in the Mathlib library, ready for us to use in our own Lean proofs. 

We'll start by using the definition of an odd number.

Task

Show the integer 13 is odd.

Maths

To show 13 is odd, we need to show it meets the definition of odd.

An odd integer is of the form $2k+1$, where $k$ is an integer. 

In more mathematical phrasing; if there exists an integer $k$ such that $n=2k+1$, then $n$ is odd.

The task has become an existence proof. If we can find an integer $k$ such that $13=2k+1$, then we have shown 13 is odd.

Let's write out a step by step proof.

$$\begin{align}13&\text{ is odd}&&\text{proof objective }\\&&&\\\exists k\in\mathbb{Z}[n=2k+1]&\implies n\text{ is odd}&&\text{definition of odd}\tag{1}\label{12.1}\\&&&\\\exists k\in\mathbb{Z}[13=2k+1]&&&\text{sufficient goal, using }(\ref{12.1})\\&&&\\\text{use }k=6&&&\text{chosen example}\tag{2}\label{12.2}\\13&=2(6)+1&&\text{using }(\ref{12.2})\\&&&\\13=2(6)+1&\implies13\text{ is odd}&&\text{by definition }(\ref{12.1})\tag*{\(\Box\)}\end{align}$$

This may look a little laborious, but the detail will help us develop a Lean proof. . 

We start with the proof objective, to show $13$ is odd.

We then state the definition, that $n$ is odd if it can be written in the form $2k+1$, where $k$ is an integer. 

So, to show $13$ is odd, it is sufficient to show it can be written in the form $2k+1$. This gives us a new goal, to show there exists an integer $k$ such that $13=2k+1$.

We choose $k=6$, and confirm that $13=2(6)+1$. 

We have shown that $13$ can indeed be written in the form $2k+1$. And so, by the definition of odd, we have finally shown that $13$ is odd.

Code

The following Lean program proves the integer 13 is odd.

-- 12 - Definition: Odd Number

import Mathlib.Tactic

example : Odd (13: ℤ)  := by
  dsimp [Odd]
  use 6
  norm_num

The proof objective states that 13 is Odd. 

If we had simply written Odd 13 as the proof objective, 13 would be interpreted, by default, as a natural number. We write (13: ℤ) to specify 13 as an integer.

The idea of Odd is defined in Mathlib. The next line dsimp [Odd] expands that definition in the Infoview so we can see what it actually is. We'll see below the goal changes from being displayed as Odd 13, to ∃ k, 13 = 2 * k + 1, which we recognise as the definition of odd applied to 13.

The instruction dsimp has no effect on the proof itself. It only changes how Odd is displayed in the Infoview. 

After this point, the proof proceeds as a simple existence proof. 

The instruction use 6 tells Lean we want to try 6 for k. This changes the goal to 13 = 2 * 6 + 1. We resolve this goal by arithmetic, using the norm_num tactic. For such a simple and clear goal, there is no need for a multi-line calc section.

What is Odd?

The proof objective was written as Odd (13: ℤ). The intention is for this to be a statement saying 13 is an odd number.

How does this work?

The idea of Odd is defined in Mathlib as a function. A definition is distinct from a lemma, and Mathlib contains many of both. A lemma needs to be justified by proof, but a definition does not.

That function Odd takes one parameter, and outputs a proposition involving that parameter, which may or may not be true.

When applied to 13, the output is a proposition  ∃k such that 13 = 2*k + 1. This proposition is true because we can prove it.

If we applied Odd to 14, the output is a proposition ∃k such that 14 = 2*k + 1. This proposition is not true because there is no proof for it.

It is interesting to see the actual definition of Odd inside Mathlib:

def Odd (a : α) : Prop := ∃ k, a = 2 * k + 1

We can see how a is mapped to a proposition about a.

Infoview

The Infoview is particularly useful when working with definitions and existence proofs.

Placing the cursor before dsimp [Odd] shows the original proof goal.

⊢ Odd 13

Moving the cursor to the end of the line after dsimp [Odd] shows the goal is now displayed using the definition of Odd.

⊢ ∃ k, 13 = 2 * k + 1

Placing the cursor after use 6 shows the goal is now specific to k = 6.

⊢ ∃ k, 13 = 2 * 6 + 1

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Easy Exercise

Write a Lean program to prove the integer 14 is even.

Your proof should use Mathlib's definition Even for even numbers. Use dsimp to see how the definition is applied to 14.