Hi, Gregbard.

I went looking for a reference for this article, but I'm not sure I can find one. The term "extendable relation" came up empty on Google Scholar. So then I ran this search and came up with 3,150 instances of "extendability" in the indexed mathematical literature.

There's no way I can wade through all 3,150 g-hits, but I did make a pass through the first 100 titles. Here are some representative titles from the list.

• Quasiconformal mappings and extendability of functions in sobolev spaces
• CR structures with group action and extendability of CR functions
• Forbidden Subgraphs and Cycle Extendability
• On the Extendability of Linear Codes
• On wedge extendability of CR-meromorphic functions
• Non-extendability of semilattice-valued measures on partially ordered sets
• Applicability and extendability of Megerlins method for solving parabolic free boundary problems
• Smooth Extendability of Proper Holomorphic Mappings

None of these (with the possible exception of the one about semilattice-valued measures) has anything to do with the notion of an involution (which the definition given in this article is all about). Most of the examples use the word "extendability" in a sense more closely related to the notion of analytic continuation – either in function spaces, or on complex manifolds, or over finite fields.

Anyway, it would be good if you could supply a reference for the definition in this article. Oh -- it also appears that "extendability" would have to be a property of a relation. and not the relation itself. DavidCBryant 12:05, 1 September 2007 (UTC) reply

I, too, have never heard of this meaning of "extendability".  -- Lambiam 21:39, 22 February 2008 (UTC) reply