Tensor Product of Lawvere Theories
The tensor product
𝒯
1
⊗
𝒯
2
tensor-product
subscript
𝒯
1
subscript
𝒯
2
\mathcal{T}_{1}\otimes\mathcal{T}_{2}
of two Lawvere
Theories is the Lawvere theory formed by considering parallel
pairs of maps
f
1
⊗
f
2
:
m
1
×
m
2
→
n
1
×
n
2
:
tensor-product
subscript
𝑓
1
subscript
𝑓
2
→
subscript
𝑚
1
subscript
𝑚
2
subscript
𝑛
1
subscript
𝑛
2
f_{1}\otimes f_{2}:m_{1}\times m_{2}\to n_{1}\times n_{2}
, quotiented to force the following diagram to commute:
m 1 × m 2 subscript 𝑚 1 subscript 𝑚 2 {{m_{1}\times m_{2}}} m 1 × n 2 subscript 𝑚 1 subscript 𝑛 2 {{m_{1}\times n_{2}}} n 1 × m 2 subscript 𝑛 1 subscript 𝑚 2 {{n_{1}\times m_{2}}} n 1 × n 2 subscript 𝑛 1 subscript 𝑛 2 {{n_{1}\times n_{2}}} m a t h r m i d i f x . . ⋄ l s e i ⊗ f 2 \scriptstyle{mathrm{id}{ifx..lse\par i}\otimes f_{2}} f 1 ⊗ m a t h r m i d i f x . . ⋄ l s e i \scriptstyle{f_{1}\otimes mathrm{id}{ifx..lse\par i}} f 1 ⊗ m a t h r m i d i f x . . ⋄ l s e i \scriptstyle{f_{1}\otimes mathrm{id}{ifx..lse\par i}} m a t h r m i d i f x . . ⋄ l s e i ⊗ f 2 \scriptstyle{mathrm{id}{ifx..lse\par i}\otimes f_{2}}
Properties
The tensor product of Lawvere theories induces a Symmetric
Monoidal structure on the Category
of Lawvere Theories , where the unit is the theory of a
set.
There is an equivalence between
Mod
(
𝒯
1
⊗
𝒯
2
,
𝒞
)
Mod
tensor-product
subscript
𝒯
1
subscript
𝒯
2
𝒞
\mathrm{Mod}(\mathcal{T}_{1}\otimes\mathcal{T}_{2},\mathcal{C})
and
Mod
(
𝒯
1
,
Mod
(
𝒯
2
,
𝒞
)
)
Mod
subscript
𝒯
1
Mod
subscript
𝒯
2
𝒞
\mathrm{Mod}(\mathcal{T}_{1},\mathrm{Mod}(\mathcal{T}_{2},\mathcal{C}))
. This makes the models of the tensor product quite odd by
the usual Eckmann-Hilton
arguments.