@InProceedings{jung93b,
  author = 	{A. Jung and J. Tiuryn},
  title = 	{A New Characterization of Lambda Definability},
  booktitle = 	{Typed Lambda Calculi and Applications},
  year = 	1993,
  series = 	{Lecture Notes in Computer Science},
  volume = 	664,
  editor = 	{M. Bezem and J. F. Groote},
  pages = 	{245--257},
  publisher = 	{Springer Verlag},
  abstract = 	{We give a new characterization of lambda definability
		  in Henkin models using logical relations defined
		  over ordered sets with varying arity. The advantage
		  of this over earlier approaches by Plotkin and
		  Statman is its simplicity and universality. Yet,
		  decidability of lambda definability for hereditarily
		  finite Henkin models remains an open problem. But if
		  the variable set allowed in terms is also restricted
		  to be finite then our techniques lead to a decision
		  procedure.}
}