Definition. Corecursive algebra

Fix an endofunctor 𝐹:π’žοΈ€β†’π’žοΈ€. An algebra 𝛼:𝐹𝐡→𝐡 is corecursive when for every coalgebra 𝛾:𝑋→𝐹𝑋 there is exactly one hylomorphism from 𝛾 to 𝛼. Equivalently, the functor π–§π—’π—…π—ˆ(βˆ’,𝛼) of the hylomorphism profunctor is constantly a singleton. It is the dual of a recursive coalgebra: an 𝐹-algebra in π’žοΈ€ is corecursive exactly when it is recursive as an 𝐹op-coalgebra in π’žοΈ€op.

Example. If (𝜈𝐹,π—ˆπ—Žπ—) is a terminal coalgebra, then π—ˆπ—Žπ— is invertible and π—ˆπ—Žπ—βˆ’1:𝐹(𝜈𝐹)β†’πœˆπΉ is a corecursive algebra: a solution of β„Ž=π›Ύβ‹†πΉβ„Žβ‹†π—ˆπ—Žπ—βˆ’1 is the same thing as a solution of β„Žβ‹†π—ˆπ—Žπ—=π›Ύβ‹†πΉβ„Ž, that is, a coalgebra map into the terminal coalgebra, and there is exactly one, π—Žπ—‡π–Ώπ—ˆπ—…π–½π›Ύ.

corecursive-algebra definition entries/category/corecursive-algebra.hel