# Library UniMath.Induction.W.Naturals

## Natural numbers

The natural numbers are a motivating example of W-types, and one of the only W-types readily available in UniMath. We show that they are an initial algebra for a polynomial functor, and satisfy a few other properties.
Author: Langston Barrett (@siddharthist)
The signature for the nat functor is (bool, true false unit) since the naturals have two constructors: one for zero and one for successor.
The functor deals with functions from ∅ and unit; these lemmas will come in handy in several proofs.
Lemma eqfromempty {X : UU} (f : empty X) : f = fromempty.
Proof. apply proofirrelevance, isapropifcontr, iscontrfunfromempty. Defined.

Lemma eta_unit {X : UU} (f : unit X) : f = λ _, f tt.
Proof. apply funextfun; intros ?; induction _; reflexivity. Defined.

Simplifying the action of the functor on arrows

Lemma nat_functor_arr_true {X Y : UU} (f : X Y) g :
(functor_on_morphisms nat_functor) f (true,, g) = (true,, fromempty).
Proof.
cbn; unfold polynomial_functor_arr; cbn.
apply maponpaths, eqfromempty.
Defined.

Lemma nat_functor_arr_false {X Y : UU} (f : X Y) g :
(functor_on_morphisms nat_functor) f (false,, g) = (false,, λ _, f (g tt)).
Proof.
cbn; unfold polynomial_functor_arr; cbn.
apply maponpaths, eta_unit.
Defined.

Here's how to prove two functions from a nat_functor are equal: check both cases.
Lemma from_nat_functor_eq {X Y : UU} :
f g : nat_functor X Y,
(f (true,, fromempty) = g (true,, fromempty))
( h, f (false,, λ _, h tt) = g (false,, λ _, h tt)) f = g.
Proof.
intros f g ? eqfalse.
apply funextsec; intro pair.
induction pair as [b bfun].
induction b.
- refine (maponpaths (λ z, f (true,, z)) (eqfromempty bfun) @ _).
refine (_ @ !maponpaths (λ z, g (true,, z)) (eqfromempty bfun)).
assumption.
- cbn in eqfalse.
refine (maponpaths (λ z, f (false,, z)) (eta_unit bfun) @ _).
refine (_ @ !maponpaths (λ z, g (false,, z)) (eta_unit bfun)).
apply eqfalse.
Defined.

The intuition is that an algebra X for this functor is given by a constant x : X and a function X → X. The following equivalence verifies this.
Definition nat_functor_equiv :
{X : UU}, (X × (X X)) (nat_functor X X).
Proof.
intro X.
use weq_iso.
× intros dprodpair.
intros pairfun.
induction pairfun as [b bfun]; induction b.
- exact (pr1 dprodpair).
- exact (pr2 dprodpair (bfun tt)).
× intro pairfun.
exact (make_dirprod (pairfun (true,, fromempty))
(λ x, pairfun (false,, λ _, x))).
× reflexivity.
× intro Y; apply from_nat_functor_eq; reflexivity.
Defined.

Using our equivalence, we can consisely define the algebra corresponding to ℕ. Any choice of zero results in an isomorphic algebra.
We may also define it directly. This will be judgmentally equal to nat_alg_z.
Example nat_alg' : algebra_ob nat_functor.
Proof.
unfold algebra_ob, nat_functor; cbn; unfold polynomial_functor_obj; cbn.
refine (,, λ pair, _).
induction pair as [b rect].
induction b; cbn in rect.
- exact 0.
- exact (S (rect tt)).
Defined.
Lemma nat_algs_eq : nat_alg_z = nat_alg'. Proof. reflexivity. Defined.

An algebra morphism between algebras for the nat functor is a function that respects all the relevant structure.
Definition make_nat_functor_algebra_mor {X Y : algebra_ob nat_functor} :
let X' := invmap nat_functor_equiv (pr2 X) in
let Y' := invmap nat_functor_equiv (pr2 Y) in
(f : pr1 X pr1 Y),
(f (pr1 X') = (pr1 Y')) × (f (pr2 X') = (pr2 Y') f)
is_algebra_mor _ X Y f.
Proof.
intros X' Y' f p.
apply from_nat_functor_eq.
+ unfold funcomp.
refine (_ @ !maponpaths _ (nat_functor_arr_true f _)).
refine (pr1 p @ _).
apply (maponpaths (pr2 Y)), maponpaths.
reflexivity.
+ intros ?; apply (eqtohomot (pr2 p)).
Defined.

Define the unique algebra morphism out of ℕ
Lemma nat_alg_is_preinitial : is_preinitial nat_alg_z.
Proof.
intro X; pose (x := pr2 X).

X has a "zero" and a "successor", just like ℕ, given by the algebra structure. We use these to define the unique morphism ℕ -> X by induction.
pose (x0 := x (true,, fromempty)).
pose (xsucc := (λ y, x (false,, λ _, y)) : pr1 X pr1 X).

The recursor for ℕ gets an extra argument of type ℕ, which we don't pass to xsucc. Compare to CategoryTheory.FunctorAlgebras.nat_ob_rec.
refine ((nat_rect _ x0 (λ _, xsucc)),, _).
apply make_nat_functor_algebra_mor; split; reflexivity.
Defined.

The first projection of the morphism out of ℕ (the actual function) is unique.
Lemma nat_alg_func_is_unique :
X, (mor : algebra_mor _ nat_alg_z X), pr1 mor = pr1 (nat_alg_is_preinitial X).
Proof.
intros X mor.
induction X as [X x]; induction mor as [mor is_mor]; cbn in x.
cbn in mor.
apply funextfun; intros n; induction n; cbn.
- unfold is_algebra_mor in is_mor; cbn in mor, is_mor.
Use the condition that mor is an algebra morphism
refine ((eqtohomot is_mor (true,, fromempty)) @ _).
apply (maponpaths x).
apply nat_functor_arr_true.
-
Use the condition that mor is an algebra morphism
refine ((eqtohomot is_mor (false,, _)) @ _); cbn.
apply (maponpaths x).
unfold polynomial_functor_arr; cbn.
apply maponpaths.
apply funextsec; intros ttt; induction ttt.
apply IHn.
Defined.

Since fibered algebras are the "dependent version" of normal algebras, we need some kind of "dependent version" of the lemmas above.
Lemma sec_fromempty {X : UU} {Y : X UU}
(f : X)
(t : z : , Y (f z)) : t = λ e, fromempty e.
Proof.
apply funextsec; intros ?; induction _.
Defined.

A fibered algebra over ℕ consists of a family ℕ → UU, a point x0 : X 0, and a function from each X n to X (S n).
Definition fibered_algebra_nat :
fibered_alg nat_alg_z (X : n : , UU), (X 0) × ( {n}, X n X (S n)).
Proof.
apply weqfibtototal; intro X; cbn in X.
use weq_iso.
- intro x.
apply make_dirprod.
+ exact (x (true,, fromempty) (λ e, fromempty e)).
+ refine (λ n xn, _).
unfold fibered_alg in x; cbn in x.
apply (x (false,, λ _, n) (λ _, xn)).
- intro x.
unfold fibered_alg; cbn.
intros pair from; induction pair as [b bfun].
induction b; cbn in ×.
+ exact (pr1 x).
+ exact (pr2 x (bfun tt) (from tt)).
- cbn. intro g.
apply funextsec; intro pair.
induction pair as [b bfun].
induction b; cbn; cbn in bfun.
+ apply funextsec; intro z.
rewrite (sec_fromempty bfun z).
rewrite (eqfromempty bfun).
reflexivity.
+ rewrite (eta_unit bfun).
apply funextsec; intro z.
apply maponpaths.
exact (!eta_unit z).
- reflexivity.
Defined.

A section from ℕ consists of a "point" x : ∏ n, X n such that x agrees with the function which is a part of the fibered algebra (see above).
Definition make_nat_alg_sec :
(FA : fibered_alg nat_alg_z),
let FA' := fibered_algebra_nat FA
in (x : n : , pr1 FA' n)
(p1 : x 0 = pr1 (pr2 FA'))
(p2 : ( n, pr2 (pr2 FA') n (x n) = x (S n))),
algebra_section FA.
Proof.
intros FA FA' x p1 p2.
unfold algebra_section.
refine (x,, _).
intro pair.
induction pair as [b bfun].
induction b; cbn; cbn in bfun.
-
They are equal at 0 by hypothesis p1, the rest is noise.
refine (p1 @ _).
unfold FA', fibered_algebra_nat; cbn.
rewrite (eqfromempty bfun).
apply maponpaths.
apply funextsec.
intro e; induction e.
- refine (!p2 (bfun tt) @ _).
rewrite (eta_unit (bfun)).
reflexivity.
Defined.

Another way to make an algebra section given different starting data.
Definition make_nat_alg_sec' {X : UU} {ρ : n, X n X (S n)}
(x : n : , X n) (H : n : , x (S n) = ρ n (x n)) :
algebra_section
(invmap fibered_algebra_nat (X,, (x 0,, ρ))).
Proof.
refine (x,, _).
intros pair; induction pair as [b bfun]; induction b.
- reflexivity.
- apply H.
Defined.

Define the section out of ℕ, prove it's really a section
Lemma nat_alg_is_preinitial_sec : is_preinitial_sec nat_alg_z.
Proof.
intro E. pose (x := pr2 E).
unfold is_preinitial, algebra_section.
use make_nat_alg_sec.
- apply nat_rect.
+ exact (x (true,, fromempty) (empty_rect (pr1 E fromempty))).
+ intros ? fn.
refine (x (false,, λ _, n) (λ b : unit, fn)).
- cbn.
apply maponpaths.
apply funextsec; intro e; induction e.
- reflexivity.
Defined.