Strong normalization from weak normalization in typed lambda-calculi

For some typed lambda-calculi it is easier to prove weak normalization than strong normalization. Techniques to infer the latter from the former have been invented over the last twenty years by Nederpelt, Klop, Khasidashvili, Karr, de Groote, and Kfoury and Wells. However, these techniques infer strong normalization of one notion of reduction from weak normalization of a more complicated notion of reduction. This paper presents a new technique to infer strong normalization of a notion of reduction in a typed lambda-calculus from weak normalization of the same notion of reduction. The technique is demonstrated to work on some well-known systems including second-order lambda-calculus and the system of positive, recursive types, It gives hope for a positive answer to the Barendregt-Geuvers conjecture stating that every pure type system which is weakly normalizing is also strongly normalizing. The paper also analyzes the relationship between the techniques mentioned above, and reviews, in less detail, other techniques for proving strong normalization. (C) 1997 academic press.