  In Chapter 3 of his Notes on constructive mathematics, Martin-L{\"o}f
describes recursively constructed ordinals. He gives a constructively
acceptable version of Kleene's computable ordinals. In fact, the Turing
definition of computable functions is not needed from a constructive point of
view. We give in this paper a constructive theory of ordinals that is similar
to Martin-L{\"o}f's theory, but based only on the two relations "x \leq y" and
"x < y", i.e. without considering sequents whose intuitive meaning is a
classical disjunction. In our setting, the operation "supremum of ordinals"
plays an important r{\^o}le through its interactions with the relations "x \leq
y" and "x < y". This allows us to approach as much as we may the notion of
linear order when the property ''\alpha \leq \beta or \beta \leq \alpha'' is
provable only within classical logic. Our aim is to give a formal definition
corresponding to intuition, and to prove that our constructive ordinals satisfy
constructively all desirable properties.

Logic