# Zulip Chat Archive

## Stream: new members

### Topic: lean baffled by my definition

#### Alexandra Foster (Jun 14 2021 at 22:21):

Hi there! I'm a math undergrad at McMaster University in Hamilton, Ontario with a bit of a geometry addiction who decided to try and acquire some skill at Lean over the course of the summer by proving some elementary propositions in algebraic geometry about monomial ideals and their corresponding varieties. However, I've hit an enigmatic error and I'm not sure what to do about it to please Lean. Here is the code I'm attempting to run:

```
import data.set.basic
import data.fintype.basic
import algebra.field
noncomputable theory
variables {k : Type} [field k]
variables {σ : Type} [fintype σ]
-- a subset V of k^n is called a "coordinate subspace"
-- if there exists a subset τ of the set of variable
-- indices σ such that
-- V = {(x₁,x₂,...,xₙ) | ∀ t ∈ τ, xₜ = 0}
def coordinate_subspace (τ : set σ) : (set (σ → k)) :=
{x : σ → k |
∀ t ∈ τ, x t = 0}
-- proposition: V is a union of coordinate subspaces
def union_of_coords (V : set (σ → k)) : Prop :=
∃ α : set (set σ),
∀ p : σ → k, p ∈ V → ∃ τ : set σ,
τ ∈ α ∧ p ∈ coordinate_subspace τ
```

The error I get:

```
-- don't know how to synthesize placeholder
-- context:
-- k : Type,
-- _inst_1 : field k,
-- σ : Type,
-- _inst_2 : fintype σ,
-- V : set (σ → k),
-- α : set (set σ),
-- p : σ → k,
-- ᾰ : p ∈ V,
-- τ : set σ
-- ⊢ Type
```

How can I uhhh... make it so that it is possible to umm... synthesize a "placeholder context"?

(BTW, for motivation & context: my eventual goal is to prove that any variety corresponding to a monomial ideal is a union of coordinate subspaces, Proposition 1 of Chapter 9 of Ideals, Varieties & Algorithms by Cox, Little & O'Shea)

#### Heather Macbeth (Jun 14 2021 at 22:23):

```
variables (k : Type) [field k]
...
-- proposition: V is a union of coordinate subspaces
def union_of_coords (V : set (σ → k)) : Prop :=
∃ α : set (set σ),
∀ p : σ → k, p ∈ V → ∃ τ : set σ,
τ ∈ α ∧ p ∈ coordinate_subspace k τ
```

#### Heather Macbeth (Jun 14 2021 at 22:23):

Lean was having trouble figuring out what the field `k`

was ... if you make that argument explicit, it can figure out what it is.

#### Heather Macbeth (Jun 14 2021 at 22:25):

(Welcome! Lots of good geometry at McMaster, although I know the differential geometry side better.)

#### Alexandra Foster (Jun 14 2021 at 22:26):

(I honestly have ambitions toward differential geometry but I sorta flunked an elementary differential geometry course last fall term so maybe my dreams are just dreams)

#### Alexandra Foster (Jun 14 2021 at 22:31):

Heather Macbeth said:

Lean was having trouble figuring out what the field

`k`

was ... if you make that argument explicit, it can figure out what it is.

all good now!! thank you!!!

Last updated: Dec 20 2023 at 11:08 UTC