Ritchie, D.M. (1979) The Evolution of the Unix Time-Sharing System. Language Design and Programming Methodology, Sydney, September 1979.
http://www.read.seas.harvard.edu/~kohler/class/aosref/ritchie84evolution.pdf

has been cited by the following article:

• TITLE: A Code Correlation Comparison of the DOS and CP/M Operating Systems

  AUTHORS: Robert Zeidman

  KEYWORDS: DOS, CP/M, Copyright Infringement, Software Forensics, Software Correlation

  JOURNAL NAME: Journal of Software Engineering and Applications, Vol.7 No.6, May 28, 2014

  ABSTRACT: For years, rumors have circulated that the code for the original DOS operating system created by Microsoft for the IBM personal computer is actually copied from the CP/M operating system developed by Digital Research Incorporated. In this paper, scientifically tested and accepted forensic analysis mathematical techniques, step-by-step processes, and advanced software code comparison tools are used to compare early versions of the two code bases. The conclusion is reached that no copying of code takes place1.