## Stream: new members

### Topic: making a well-defined definition

#### Ali Sever (Aug 05 2018 at 08:13):

I have two sets A B and I want to define f A B = g A b(another set) for some b ∈ B. I have the fact that ∃ b, b ∈ B, but I'm not allowed to use Exists.dcases_on. Also, how/when do I prove this is well-defined and doesn't depend on the choice of b?

#### Chris Hughes (Aug 05 2018 at 08:49):

You can use classical.some in which case you don't have to prove that it's well defined. Alternatively, if the possible values for B form a partition, you could use quotients.

#### Ali Sever (Aug 05 2018 at 08:50):

I can prove that every value gives the same result.

#### Chris Hughes (Aug 05 2018 at 08:56):

Presumably then, it isn't defined or at least used on any sets, just sets that meet some criteria?

Yep, (lines)

#### Chris Hughes (Aug 05 2018 at 09:15):

If you want it to be defined on the set you pretty much have to use classical.some

#### Kevin Buzzard (Aug 05 2018 at 09:15):

You can't get data (the set) from a proof (the fact that there exists a b) constructively -- even if you can prove uniqueness results. I've been using cases (classical.indefinite_description _ H) with b Hb to get b out of the hypothesis H that b exists, recently, but I found a glitch with this idiom recently; it doesn't always do quite what I want it to. Might work here though.

#### Kevin Buzzard (Aug 05 2018 at 09:18):

The problem is that if you imagine a function as a computer program, the proof that a number exists (like Waring's number G(3) in my Monday talk) doesn't give you the right to be able to compute it for free.

#### Chris Hughes (Aug 05 2018 at 09:22):

A lot of the time when you know something exists, you can compute it however, because that's how you proved it exists. I'm guessing Ali might be in this situation.

#### Ali Sever (Aug 05 2018 at 09:23):

I just want to use the existence to show the set is non-empty. I think I might have to define the thing I want in a different way, and show it's equal to what I wanted.

#### Kevin Buzzard (Aug 05 2018 at 10:20):

Computing the set can't be done constructively, but I should think that proving it's non-empty might be possible. If your goal is the assertion that some set is non-empty then you'll be able to uses cases on the exists normally, because your goal is to prove a proposition, not to construct some data.

Last updated: May 18 2021 at 17:44 UTC