## The Wonderful World of Realizability, part 2

### Peter Johnstone – 1 November 2017

Given the great variety of Schonfinkel algebras, it becomes a matter of some importance to gain an understanding of the geometric morphisms between the toposes which they generate -- in other words, to form a (2-)category of Schonfinkel algebras and to try to understand its structure. The first serious steps in this direction were taken by John Longley in his Edinburgh PhD thesis (1994), but the final piece of the jigsaw was fitted into place only in 2013. In this talk I'll trace the development of our understanding.

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