845 **McCarthy’s 91-function: an unfortunate paradigm**

Nuenen, 21 December 1983. 2 pages. transcription.

McCarthy’s 91-function for recursively defined functions. An ingenious construction, but an unfortunate paradigm. A demonstration that is both lengthy and boring, as advertised.

—JDA

853a **Ingenieur Ebergen, Weledelgestrengde Heer **transcription

The text is an open letter to a doctoral student of Dijkstra’s. Having spent a semester as assistant at the California Institute of Technology in Pasadena, the student wanted to pursue the subject he was assigned to there as his thesis subject. Dijkstra accepted but had his doubts: he had given him a perfectly outlined subject he was certain would have been completed much faster.

However, the student always seemed comfortable with his chosen subject and, after a terrible first draft of his thesis, he presented a second draft that was almost equal to its final form. Dijkstra believes the initial subject would have been much easier and would have produced a “more convincing piece” but also recognizes that the student has learned much more with his chosen subject, having had to introduce new terms and notations.

—LJ

866 **An analytical proof of The Butterfly Theorem**

Nuenen, 14 November 1983. 3 pages. transcription.

Text is the author's own proof of the butterfly quadrilateral inscribed inside a circle. A long proof, but a nice exercise in analytical geometry.

—JDA

870 **My mother’s proof of the Butterfly Theorem (see EWD866)**

Nuenen, 18 December 1983. 2 pages. transcription.

Text is his mother's proof of the butterfly quadrilateral inscribed inside a circle, using conic sections, which is far superior to his own proof using line bundles. Bless her heart!

—JDA