In the preface of each of his books and on his websiteSee http://sunburn.stanford.edu/~knuth/books.html, computer scientist Donald Knuth offers to cheerfully pay a reward of "$2.56" (USD) to the first finder of each error, whether it be technical, typographical, or historical. Knuth explains that $2.56, or 256 cents, correspond to one hexadecimal dollar.Frequently Asked Questions on Don Knuth's webpage Valuable suggestions are worth 32¢.

These reward checks have been described as "among computerdom's most prized trophies".Steve Ditlea . Rewriting the Bible in 0's and 1's. ''MIT's "Technology Review", 11 January 2002. As of October 2001, Knuth reports having written more than 2,000 such checks, with an average value exceeding $8 per check.http://www.ams.org/notices/200203/fea-knuth.pdf As of March 2005, the total value of the checks signed by Knuth was over $20,000 (see NPR interview below). Very few of these checks are actually cashed, however, even the largest ones; more often, they are framed, or kept as bragging rights.Kara Platoni, Love at First Byte. Stanford Magazine, May-June 2006The History of TeX

Intelligence: Finding an error in a Knuth text.

Stupidity: Cashing that $2.56 check you got.

Seen in a Slashdot signature, quoted by Edward O'Connorhttp://www.stgray.com/quotes/programming.html

The reward for coding errors found in Knuth's TeX and METAFONT programs (as distinguished from errors in Knuth's books) followed an audacious scheme inspired by the Wheat and Chessboard Problem . It started at $2.56, and doubled every year until it reached $327.68 http://www.ams.org/notices/200203/fea-knuth.pdf. Recipients of this "sweepstakes" reward include Chris Thompson (Cambridge) and Boguslaw Jackowski (Gdansk)http://www.uni-giessen.de/hrz/tex/more_info/info/mailarchiv/mutex.1995/msg00147.html.

Knuth is often not able to answer immediately when a reader finds a mistake in one of his books or programs; in some cases, the delay was several years long. For example, on 1 July, 1996, Knuth sent out more than 250 letters, 125 of which contained checks, for errors reported in The Art of Computer Programming since the summer of 1981. A few of these remain unclaimed as of May 2006.What is your current mailing address? on Don Knuth's website. When Knuth is not able to reply immediately, he adds a 5% interest, compounded continuously, to the reward.See http://www.math.rutgers.edu/~zeilberg/mamarim/mamarimPS/king.ps (PostScript file), and http://developers.slashdot.org/comments.pl?sid=29586&pid=3178058#3178872.

Each check's memo field identifies the book and page number. 1.23 indicates an error on page 23 of Volume 1. (1.23) indicates a valuable suggestion on that page. The symbol Θ denotes the book Things a Computer Scientist Rarely Talks About, KLR denotes the book Mathematical Writing (by Knuth, Larrabee, and Roberts), GKP and CM denote the book Concrete Mathematics (by Graham, Knuth, and Patashnik), f1 denotes fascicle 1, and CMT denotes the book Computer Modern Typefaces.

Known checks

Frank Ruskey's class with checks

Notes and references