Corecursion exploits the existence of a morphism from a coalgebra for an endofunctor to a terminal coalgebra for the same endofunctor to define an operation. It is dual to recursion. See also coinduction.


  1. For the endofunctor H(X)=1+XH(X) = 1 + X on Set, the terminal coalgebra is ¯\bar{\mathbb{N}}, the extended natural number system. Define a function add:¯×¯1+¯×¯add\colon \bar{\mathbb{N}} \times \bar{\mathbb{N}} \to 1 + \bar{\mathbb{N}} \times \bar{\mathbb{N}}:

    add(n,m)={(pred(n),m) ifn>0; (0,pred(m)) ifn=0,m>0; * ifm=n=0, add(n, m) = \begin{cases} (pred(n), m) & if\; n \gt 0; \\ (0, pred(m)) & if\; n = 0,\; m \gt 0; \\ * & if\; m = n = 0, \end{cases}

    where pred(x)pred(x) is as defined at extended natural number.

    Then (¯×¯,add)(\bar{\mathbb{N}} \times \bar{\mathbb{N}}, add) is an HH-coalgebra. The unique coalgebra morphism +:¯×¯¯{+}\colon \bar{\mathbb{N}} \times \bar{\mathbb{N}} \to \bar{\mathbb{N}} (to the terminal coalgebra ¯\bar{\mathbb{N}}) is addition on the extended natural numbers.


  • Jiří Adámek, Introduction to Coalgebra (pdf)

  • Lawrence Moss, Norman Danner, On the Foundations of Corecursion (journal, pdf)

Revised on September 28, 2012 16:18:36 by Urs Schreiber (