Electronic textbooks of mathematics would benefit from interactively explorable proofs, where the reader can request a more detailed explanation for any part of the proof he or she does not understand. This requires that definitions, theorem statements, and proofs are written in some formal language. In this thesis we investigate the theoretical and practical challenges of producing such textbooks.

Any particular choice of language cannot be adequate for all textbooks, so we construct a theoretical framework for discussing extensible languages. Under this framework we define three languages for expressing definitions and theorem statements. The first two correspond to the standard languages of propositional and first-order logics. The third language is expressive enough for most definitions and theorem statements in discrete mathematics, but conceptually less problematic than the languages of set theory and higher-order logic, because it cannot express unrestricted quantification over sets that are larger than the set of real numbers.

In addition, an implementation of an interactive proof explorer is presented. Its capabilities are demonstrated with an explorable proof of Bertrand’s postulate. The focus of this thesis is on the experience of the reader, and the results seem promising in that regard. With further work on making the authoring tools more efficient to use, it should be feasible to formalize an entire textbook.