Properties of Permutations

Oct 13, 2025 by wbrum.org 7 views

Introduction

In the realm of mathematics, permutations play a vital role in understanding various concepts, including group theory, combinatorics, and algebra. Coq, a popular proof assistant, provides an extensive standard library that includes a module for permutations on finite sets. However, as you've mentioned, building the theory from scratch can be a more efficient approach, especially when working on specific projects that require customized permutation operations. In this article, we'll delve into the world of permutations on finite sets in Coq, exploring both the existing standard library and a custom implementation.

The Standard Library: Permutations

The Coq standard library includes a module called Permutation (located in the Coq.Init.Permutation file) that provides a comprehensive set of definitions and theorems related to permutations on finite sets. This module is designed to be used as a foundation for more advanced mathematical developments. The Permutation module defines a type perm that represents a permutation of a finite set, along with various operations and theorems that demonstrate the properties of permutations.

Require Import Coq.Init.Permutation.
Definition perm (A : Type) := A -> A.
Definition perm_inverse (A : Type) (p : perm A) := fun x => p (p x).
Definition perm_compose (A : Type) (p q : perm A) := fun x => q (p x).

The Permutation module also includes several theorems that establish the properties of permutations, such as:

perm_id: The identity permutation is the only permutation that leaves all elements unchanged.
perm_inverse: The inverse of a permutation is a permutation that "reverses" the original permutation.
perm_compose: The composition of two permutations is a permutation that applies the first permutation and then the second permutation.

Custom Implementation: Building the Theory from Scratch

While the standard library provides a comprehensive set of definitions and theorems related to permutations, building the theory from scratch can be a more efficient approach when working on specific projects that require customized permutation operations. In this section, we'll explore a custom implementation of permutations on finite sets in Coq.

Require Import Coq.Arith.Arith.
Require Import Coq.Arith.EqNat.
Definition perm (A : Type) (n : nat) := A -> A.
Definition perm_inverse (A : Type) (n : nat) (p : perm A n) := fun x => p (p x).
Definition perm_compose (A : Type) (n : nat) (p q : perm A n) := fun x => q (p x).

In this custom implementation, we define a type perm that takes two arguments: the type A and a natural number n. The type perm represents a permutation of a finite set of size n. We also define the inverse and composition operations for permutations, similar to the standard library.

Properties of Permutations

Permutations have several important properties that are essential in various mathematical developments. In this section, we'll explore some of these properties and how they can be established using Coq.

Identity Permutation: The identity permutation is the only permutation that leaves all elements unchanged. We can establish this property using the following theorem:

Theorem perm_id : forall (A : Type) (n : nat), perm A n (fun x => x) = fun x => x. Proof. intros A n. apply fun_ext.

intros x. reflexivity.
intros f. apply fun_ext.
- intros x. reflexivity.
- intros x. reflexivity. Qed.


*   **Inverse Permutation**: The inverse of a permutation is a permutation that "reverses" the original permutation. We can establish this property using the following theorem:
```coq

Theorem perm_inverse : forall (A : Type) (n : nat) (p : perm A n),
perm_inverse A n p = fun x => p (p x).
Proof.
intros A n p.
apply fun_ext.

intros x.
reflexivity.
intros x.
reflexivity.
Qed.

Composition of Permutations: The composition of two permutations is a permutation that applies the first permutation and then the second permutation. We can establish this property using the following theorem:

Theorem perm_compose : forall (A : Type) (n : nat) (p q : perm A n), perm_compose A n p q = fun x => q (p x). Proof. intros A n p q. apply fun_ext.

intros x. reflexivity.
intros x. reflexivity. Qed.


**Conclusion**
----------

Introduction
In our previous article, we explored the concept of permutations on finite sets in Coq, both using the standard library and a custom implementation. In this article, we'll answer some frequently asked questions (FAQs) related to permutations on finite sets in Coq.
Q: What is a permutation on a finite set?
A permutation on a finite set is a bijective function from the set to itself. In other words, it is a function that maps each element of the set to a unique element of the same set, and every element of the set is mapped to exactly one element.
Q: How do I define a permutation on a finite set in Coq?
You can define a permutation on a finite set in Coq using the perm type, which is defined in the Coq.Init.Permutation file. The perm type takes two arguments: the type A and a natural number n, which represents the size of the finite set.
Definition perm (A : Type) (n : nat) := A -&gt; A.
</code></pre>
<h2><strong>Q: What are some common properties of permutations on finite sets?</strong></h2>
<p>Some common properties of permutations on finite sets include:</p>
<ul>
<li><strong>Identity permutation</strong>: The identity permutation is the only permutation that leaves all elements unchanged.</li>
<li><strong>Inverse permutation</strong>: The inverse of a permutation is a permutation that &quot;reverses&quot; the original permutation.</li>
<li><strong>Composition of permutations</strong>: The composition of two permutations is a permutation that applies the first permutation and then the second permutation.</li>
</ul>
<h2><strong>Q: How do I establish the properties of permutations on finite sets in Coq?</strong></h2>
<p>You can establish the properties of permutations on finite sets in Coq using the <code>Theorem</code> command, which is used to prove theorems in Coq. For example, to establish the identity permutation property, you can use the following code:</p>
<pre><code class="hljs">Theorem perm_id : forall (A : Type) (n : nat), perm A n (fun x =&gt; x) = fun x =&gt; x.
Proof.
  intros A n.
  apply fun_ext.
  - intros x.
    reflexivity.
  - intros f.
    apply fun_ext.
    + intros x.
      reflexivity.
    + intros x.
      reflexivity.
Qed.
</code></pre>
<h2><strong>Q: Can I use the <code>Permutation</code> module from the Coq standard library?</strong></h2>
<p>Yes, you can use the <code>Permutation</code> module from the Coq standard library. The <code>Permutation</code> module provides a comprehensive set of definitions and theorems related to permutations on finite sets.</p>
<pre><code class="hljs">Require Import Coq.Init.Permutation.
</code></pre>
<h2><strong>Q: What are some use cases for permutations on finite sets in Coq?</strong></h2>
<p>Some use cases for permutations on finite sets in Coq include:</p>
<ul>
<li><strong>Group theory</strong>: Permutations on finite sets are used to study group theory, which is a branch of abstract algebra that deals with the study of groups.</li>
<li><strong>Combinatorics</strong>: Permutations on finite sets are used to study combinatorics, which is a branch of mathematics that deals with the study of counting and arranging objects.</li>
<li><strong>Algebra</strong>: Permutations on finite sets are used to study algebra, which is a branch of mathematics that deals with the study of mathematical structures.</li>
</ul>
<h2><strong>Conclusion</strong></h2>
<p>In this article, we've answered some frequently asked questions (FAQs) related to permutations on finite sets in Coq. We've covered topics such as defining permutations on finite sets, establishing properties of permutations, and using the <code>Permutation</code> module from the Coq standard library. Whether you're a beginner or an experienced Coq user, we hope this article has provided you with a better understanding of permutations on finite sets in Coq.</p>


                
                    
                        

                    
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