# Zulip Chat Archive

## Stream: new members

### Topic: iff raises error

#### Jason KY. (May 22 2020 at 18:52):

I have the following #mwe

```
import topology.basic
variables {X : Type*} [topological_space X]
variables {Y : Type*} [topological_space Y]
def is_continuous (f : X → Y) : Prop :=
∀ U : set Y, is_open U → is_open (f ⁻¹' U)
structure topological_space_equiv
(X Y) [topological_space X] [topological_space Y] extends X ≃ Y :=
(contin : is_continuous to_fun)
(inv_contin : is_continuous inv_fun)
notation X ` ≃* ` Y := topological_space_equiv X Y
open function
noncomputable theory
example : (X ≃* Y) →
(∃ (f : X → Y) (h₀ : bijective f) (h₁ : is_continuous f),
∀ U : set X, is_open U → is_open (f '' U)) := sorry -- no errors
example : (∃ (f : X → Y) (h₀ : bijective f) (h₁ : is_continuous f),
∀ U : set X, is_open U → is_open (f '' U)) → (X ≃* Y) := sorry -- no errors
example : (X ≃* Y) ↔ -- breaks here
(∃ (f : X → Y) (h₀ : bijective f) (h₁ : is_continuous f),
∀ U : set X, is_open U → is_open (f '' U)) := sorry
```

As you can see, the first two examples work fine but the last one raises the error

```
type mismatch at application
iff (X ≃* Y)
term
X ≃* Y
has type
Type (max u_1 u_2) : Type (max (u_1+1) (u_2+1))
but is expected to have type
Prop : Type
```

Why might this be?

#### Kenny Lau (May 22 2020 at 18:53):

yeah, unlike in maths, "X and Y are homeomorphic" isn't a proposition

#### Jason KY. (May 22 2020 at 18:55):

Oh! I see! I should've thought about it a bit more

#### Kevin Buzzard (May 22 2020 at 18:55):

`equiv X Y`

is data. That's why it's more than `bijective f`

(which is a Prop)

#### Patrick Massot (May 22 2020 at 18:56):

Kenny Lau said:

yeah, unlike in maths, "X and Y are homeomorphic" isn't a proposition

That's not the issue. The issue is a somewhat confusing notation for the type of homeomorphisms from X to Y

#### Jason KY. (May 22 2020 at 18:56):

What's wrong with the notation?

#### Patrick Massot (May 22 2020 at 18:57):

It suggests a Prop

#### Patrick Massot (May 22 2020 at 18:57):

I found it very confusing in the beginning (the basic equiv case, this has nothing to do with topology) but I got used to it

#### Jason KY. (May 22 2020 at 18:58):

I think it's just confusing the first time it comes up. Hopefully I won't ever assume its a prop anymore :)

Last updated: May 13 2021 at 06:15 UTC