How did Dennis Ritchie Produce his PhD Thesis? A Typographical Mystery David F. Brailsford School of Computer Science University of Nottingham Nottingham NG8 1BB, UK dfb@cs.nott.ac.uk Brian W. Kernighan Department of Computer Science Princeton University Princeton, NJ 08540, USA bwk@cs.princeton.edu William A. Ritchie Thinkfun Inc. 1725 Jamieson Ave Alexandria, VA 22314 bill.ritchie@thinkfun.com ABSTRACT Dennis Ritchie, the creator of the C programming language and, with Ken Thompson, the co-creator of the Unix operat- ing system, completed his Harvard PhD thesis on recursive function theory in early 1968. But for unknown reasons, he never officially received his degree, and the thesis itself dis- appeared for nearly 50 years. This strange set of circum- stances raises at least three broad questions: • What was the technical contribution of the thesis? • Why wasn’t the degree granted? • How was the thesis prepared? This paper investigates the third question: how was a long and typographically complicated mathematical thesis pro- duced at a very early stage in the history of computerized document preparation? CCS CONCEPTS I.7 [Document and Text Processing]: I.7.1: Document and Text Editing, I.7.2: Document Preparation. KEYWORDS mathematical typesetting, electric typewriter, digital restora- tion, archiving, troff, Postscript fonts, IBM 2741 ACM Reference Format David F. Brailsford, Brian W. Kernighan and William A. Ritchie. 2022. How did Dennis Ritchie Produce his PhD Thesis? A Typo- graphical Mystery. In Proceedings of The 22nd ACM Symposium on Document Engineering (DocEng2022). ACM, New York, NY, USA, 10 pages. https://doi.org/10.1145/3558100.3563839 1. Introduction In June 2020, David Brock, a historian of technology and director of the Computer History Museum’s Software His- tory Center, published Discovering Dennis Ritchie’s Lost Permission to make digital or hard copies of all or part of this work for per- sonal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. For all other uses, contact the authors. DocEng2022, Sept 2022, Virtual Event, CA USA. © Copyright held by the authors 978-1-4503-1789-4/13/09. https://doi.org/10.1145/3558100.3563839 Dissertation, an article about Dennis’s long-lost PhD thesis, Program Structure and Computational Complexity. Brock’s article [1] makes for fascinating reading. Much of it is focused on the thesis’s contributions to recursive function theory and early theoretical computer science. To over-sim- plify, the thesis showed that a class of programs expressed as assignments, increments, and nested loops was capable of performing arbitrary computations. Quoting Brock, “In loop programs, one can set a variable to zero, add 1 to a variable, or move the value of one variable to another. That’s it. The only control available in loop programs is ... a simple loop, in which an instruction sequence is repeated a certain num- ber of times. Importantly, loops can be "nested," that is, loops within loops.” In more modern terms, these loop pro- grams are a Turing-complete computational model, equiva- lent to Turing machines and Church’s lambda calculus. The first section of Brock’s article, "Everything but Bound Copy," explores an intriguing open question. Although the thesis was essentially finished, lacking only a handful of triv- ial typographical corrections and presumably a pro forma final public oral exam, the thesis was never submitted to Harvard (or so it is believed), it definitely was never accepted by Harvard, and thus Dennis never actually received his PhD. Why wasn’t the thesis accepted by Harvard? Why didn’t Dennis ever get his PhD? Indeed, why did he never explic- itly acknowledge the unusual situation? And how did the thesis simply disappear for nearly 50 years, coming to light only after Dennis’s sister Lynn tracked down a copy after his death in October 2011? One oft-told story was that Harvard wanted a fee for pro- cessing the thesis and Dennis thought that he shouldn’t have to pay it. If thesis rejection was as simple as a library fee dispute, however, we should expect that Dennis would have recounted the story, embraced it in his typically self-depre- cating way, and turned it into a life lesson similar to the way he described how he wasn’t smart enough to become a physicist so he turned to computing. Instead, the thesis disappeared for 50 years and was never mentioned by Dennis. More significant is how he allowed the uncorrected story that he had a doctorate to spread 1