Y is a least fixed point combinator

The theory of recursive functions is related in a well-known way to the notion of least fixed points, by endowing a set of partial functions with an ordering in terms of their domain of definition. When terms in the pure lambda-calculus are considered as partial functions on the set of reduced lambda-terms, they inherit such a partial order. We prove that Curry’s well-known fixed point combinator Y produces least fixed points with respect to this partial order.