# mathlibdocumentation

group_theory.eckmann_hilton

# Eckmann-Hilton argument #

The Eckmann-Hilton argument says that if a type carries two monoid structures that distribute over one another, then they are equal, and in addition commutative. The main application lies in proving that higher homotopy groups (πₙ for n ≥ 2) are commutative.

## Main declarations #

• eckmann_hilton.comm_monoid: If a type carries a unital magma structure that distributes over a unital binary operation, then the magma is a commutative monoid.
• eckmann_hilton.comm_group: If a type carries a group structure that distributes over a unital binary operation, then the group is commutative.
structure eckmann_hilton.is_unital {X : Type u} (m : X → X → X) (e : X) :
Prop
• to_is_left_id : m e
• to_is_right_id : m e

is_unital m e expresses that e : X is a left and right unit for the binary operation m : X → X → X.

theorem eckmann_hilton.one {X : Type u} {m₁ m₂ : X → X → X} {e₁ e₂ : X} (h₁ : e₁) (h₂ : e₂) (distrib : ∀ (a b c d : X), m₁ (m₂ a b) (m₂ c d) = m₂ (m₁ a c) (m₁ b d)) :
e₁ = e₂

If a type carries two unital binary operations that distribute over each other, then they have the same unit elements.

In fact, the two operations are the same, and give a commutative monoid structure, see eckmann_hilton.comm_monoid.

theorem eckmann_hilton.mul {X : Type u} {m₁ m₂ : X → X → X} {e₁ e₂ : X} (h₁ : e₁) (h₂ : e₂) (distrib : ∀ (a b c d : X), m₁ (m₂ a b) (m₂ c d) = m₂ (m₁ a c) (m₁ b d)) :
m₁ = m₂

If a type carries two unital binary operations that distribute over each other, then these operations are equal.

In fact, they give a commutative monoid structure, see eckmann_hilton.comm_monoid.

theorem eckmann_hilton.mul_comm {X : Type u} {m₁ m₂ : X → X → X} {e₁ e₂ : X} (h₁ : e₁) (h₂ : e₂) (distrib : ∀ (a b c d : X), m₁ (m₂ a b) (m₂ c d) = m₂ (m₁ a c) (m₁ b d)) :
m₂

If a type carries two unital binary operations that distribute over each other, then these operations are commutative.

In fact, they give a commutative monoid structure, see eckmann_hilton.comm_monoid.

theorem eckmann_hilton.mul_assoc {X : Type u} {m₁ m₂ : X → X → X} {e₁ e₂ : X} (h₁ : e₁) (h₂ : e₂) (distrib : ∀ (a b c d : X), m₁ (m₂ a b) (m₂ c d) = m₂ (m₁ a c) (m₁ b d)) :
m₂

If a type carries two unital binary operations that distribute over each other, then these operations are associative.

In fact, they give a commutative monoid structure, see eckmann_hilton.comm_monoid.

def eckmann_hilton.add_comm_monoid {X : Type u} {m₁ : X → X → X} {e₁ : X} (h₁ : e₁) [h : add_zero_class X] (distrib : ∀ (a b c d : X), m₁ (a + b) (c + d) = m₁ a c + m₁ b d) :

If a type carries a unital additive magma structure that distributes over a unital binary operations, then the additive magma structure is a commutative additive monoid.

Equations
def eckmann_hilton.comm_monoid {X : Type u} {m₁ : X → X → X} {e₁ : X} (h₁ : e₁) [h : mul_one_class X] (distrib : ∀ (a b c d : X), m₁ (a * b) (c * d) = m₁ a c * m₁ b d) :

If a type carries a unital magma structure that distributes over a unital binary operations, then the magma structure is a commutative monoid.

Equations
def eckmann_hilton.comm_group {X : Type u} {m₁ : X → X → X} {e₁ : X} (h₁ : e₁) [G : group X] (distrib : ∀ (a b c d : X), m₁ (a * b) (c * d) = m₁ a c * m₁ b d) :

If a type carries a group structure that distributes over a unital binary operation, then the group is commutative.

Equations
def eckmann_hilton.add_comm_group {X : Type u} {m₁ : X → X → X} {e₁ : X} (h₁ : e₁) [G : add_group X] (distrib : ∀ (a b c d : X), m₁ (a + b) (c + d) = m₁ a c + m₁ b d) :

If a type carries an additive group structure that distributes over a unital binary operation, then the additive group is commutative.

Equations