Cauchy series of species of types

Content created by Fredrik Bakke, Egbert Rijke and Victor Blanchi.

Created on 2023-04-08.
Last modified on 2024-02-06.

module species.cauchy-series-species-of-types where
open import foundation.cartesian-product-types
open import foundation.dependent-pair-types
open import foundation.equivalences
open import foundation.functoriality-cartesian-product-types
open import foundation.functoriality-dependent-pair-types
open import foundation.postcomposition-functions
open import foundation.universe-levels

open import species.species-of-types


In classical mathematics, the Cauchy series of a species (of finite types) S is the formal series in x :

Σ (n : ℕ) (|S({1,...,n}| x^n / n!))

The categorified version of this series is :

  Σ (F : 𝔽), S(F) × (F → X)

Remarks that we can generalized this to species of types with the following definition :

  Σ (U : UU), S(U) × (U → X)


cauchy-series-species-types :
  {l1 l2 l3 : Level}  species-types l1 l2  UU l3  UU (lsuc l1  l2  l3)
cauchy-series-species-types {l1} S X = Σ (UU l1)  U  S U × (U  X))


Equivalent species of types have equivalent Cauchy series

module _
  {l1 l2 l3 l4 : Level}
  (S : species-types l1 l2)
  (T : species-types l1 l3)
  (f : (F : UU l1)  (S F  T F))
  (X : UU l4)

  equiv-cauchy-series-equiv-species-types :
    cauchy-series-species-types S X  cauchy-series-species-types T X
  equiv-cauchy-series-equiv-species-types =
    equiv-tot λ X  equiv-product (f X) id-equiv

Cauchy series of types are equivalence invariant

module _
  {l1 l2 l3 l4 : Level}
  (S : species-types l1 l2)
  (X : UU l3)
  (Y : UU l4)
  (e : X  Y)

  equiv-cauchy-series-species-types :
    cauchy-series-species-types S X  cauchy-series-species-types S Y
  equiv-cauchy-series-species-types =
    equiv-tot  F  equiv-product id-equiv (equiv-postcomp F e))

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