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This is the year when I promised to do the seven-year cleanup of TeX and METAFONT (last updated in 2014). I hope to have that completed soon.

`VIRUS - VIRES - FIRES - FIRER - FIVER - FEVER`

(fortunately my family and I are still healthy)

The fourth volume of The Art of Computer Programming deals with Combinatorial Algorithms, the area of computer science where good techniques have the most dramatic effects. (I love it the most, because one good idea can often make a program run a million times faster.) It's a huge, fascinating subject, and I published Part 1 (Volume 4A, 883 pages, now in its fifteenth printing) in 2011.

Two-thirds of Part 2 (Volume 4B) are now available in preliminary paperback form as Volume 4, Fascicle 5 (v4f5): “Mathematical Preliminaries Redux; Introduction to Backtracking; Dancing Links”; and Volume 4, Fascicle 6 (v4f6): “Satisfiability”. Here are excerpts from the hype on the back cover of v4f5 (382 pages):

This fascicle, brimming with lively examples, forms the first third of what will eventually become hardcover Volume 4B. It begins with a 27-page tutorial on the major advances in probabilistic methods that have been made during the past 50 years, since those theories are the key to so many modern algorithms. Then it introduces the fundamental principles of efficient backtrack programming, a family of techniques that have been a mainstay of combinatorial computing since the beginning. This introductory material is followed by an extensive exploration of important data structures whose links perform delightful dances.

That section unifies a vast number of combinatorial algorithms by showing that they are special cases of the general XCC problem --- “exact covering with colors.” The firstfruits of the author's decades-old experiments with XCC solving are presented here for the first time, with dozens of applications to a dazzling array of questions that arise in amazingly diverse contexts.

The utility of this approach is illustrated by showing how it resolves and extends a wide variety of fascinating puzzles, old and new. Puzzles provide a great vehicle for understanding basic combinatorial methods and fundamental notions of symmetry. The emphasis here is on how to create new puzzles, rather than how to solve them. A significant number of leading computer scientists and mathematicians have chosen their careers after being inspired by such intellectual challenges. More than 650 exercises are provided, arranged carefully for self-instruction, together with detailed answers---in fact, sometimes also with answers to the answers.

And here is the corresponding hype on the back cover of v4f6 (310 pages, to appear soon in its fifth printing):

This fascicle, brimming with lively examples, introduces and surveys “Satisfiability,” one of the most fundamental problems in all of computer science: Given a Boolean function, can its variables be set to at least one pattern of 0s and 1 that will make the function true?

Satisfiability is far from an abstract exercise in understanding formal systems. Revolutionary methods for solving such problems emerged at the beginning of the twenty-first century, and they've led to game-changing applications in industry. These so-called “SAT solvers” can now routinely find solutions to practical problems that involve millions of variables and were thought until very recently to be hopelessly difficult.

Fascicle 6 presents full details of seven different SAT solvers, ranging from simple algorithms suitable for small problems to state-of-the-art algorithms of industrial strength. Many other significant topics also arise in the course of the discussion, such as bounded model checking, the theory of traces, Las Vegas algorithms, phase changes in random processes, the efficient encoding of problems into conjunctive normal form, and the exploitation of global and local symmetries. More than 500 exercises are provided, arranged carefully for self-instruction, together with detailed answers.

I worked particularly hard while preparing many of the new exercises, attempting to improve on expositions that I found in the literature; and in several noteworthy cases, nobody has yet pointed out any errors. It would be nice to believe that I actually got the details right in my first attempt. But that seems unlikely, because I had hundreds of chances to make mistakes. So I fear that the most probable hypothesis is that nobody has been sufficiently motivated to check these things out carefully as yet.

I still cling to a belief that these details are extremely instructive, and I'm uncomfortable with the prospect of printing a hardcopy edition with so many exercises unvetted. Thus I would like to enter here a plea for some readers to tell me explicitly, “Dear Don, I have read exercise N and its answer very carefully, and I believe that it is 100% correct,” where N is one of the following exercises in Volume 4 Fascicle 5:

- MPR-28-29: Prove basic inequalities for sums of independent binary random variables
- MPR-50: Prove that Ross's conditional expectation inequality is sharper than the second moment inequality
- MPR-59: Derive the four functions theorem
- MPR-61: Show that independent binary random variables satisfy the FKG inequality
- MPR-99: Generalize the Karp–Upfal–Wigderson bound on expected loop iterations
- MPR-103-104: Study ternary “coupling from the past”
- MPR-114: Prove Alon's “combinatorial nullstellensatz”
- MPR-121-122: Study the Kullback–Leibler divergence of one random variable from another
- MPR-127: Analyze the XOR of independent sparse binary vectors
- MPR-130-131: Derive paradoxical facts about the Cauchy distribution (which has “heavy tails”)
- 7.2.2-75-76: Devise an algorithm that lists every
*n*-element connected subset of a given graph - 7.2.2-79: Analyze the sounds that are playable on the pipe organ in my home
- 7.2.2.1-29-30: Characterize all search trees that can arise with Algorithm X
- 7.2.2.1-53: Find every 4-clue instance of shidoku (4×4 sudoku)
- 7.2.2.1-55: Determine the fewest clues needed to force highly symmetric sudoku solutions
- 7.2.2.1-69: Investigate gerrymandering in Bitland
- 7.2.2.1-91: Find the longest right word stairs in
`WORDS`(1000) and the longest left word stairs in`WORDS`(500) - 7.2.2.1-103: List all of the 12-tone rows with the all-interval property, and study their symmetries
- 7.2.2.1-104: Construct infinitely many “perfect”
*n*-tone rows - 7.2.2.1-109: Encode any given “wordcross puzzle” as an XCC problem
- 7.2.2.1-115: Find all hypersudoku solutions that are symmetric under transposition or under 90° rotation
- 7.2.2.1-121: Determine which of the 92 Wang tiles in exercise 2.3.4.3–5 can actually be used when tiling the whole plane
- 7.2.2.1-129: Enumerate all the symmetrical solutions to MacMahon's triangle-tiling problem
- 7.2.2.1-147: Construct all of the “bricks” that can be made with MacMahon's 30 six-colored cubes
- 7.2.2.1-151-152: Arrange all of the path dominoes into a single loop
- 7.2.2.1-172: Find the longest snake-in-the-box paths and cycles that can be made by kings, queens, rooks, bishops, or knights on a chessboard
- 7.2.2.1-189: Determine the asymptotic behavior of the Gould numbers
- 7.2.2.1-196: Analyze the running time of Algorithm X on bounded permutation problems
- 7.2.2.1-215: Show that exclusion of noncanonical bipairs can yield a dramatic speedup
- 7.2.2.1-262: Study the ZDDs for domino and diamond tilings that tend to have large “frozen” regions
- 7.2.2.1-305-306: Find optimum arrangements of the windmill dominoes
- 7.2.2.1-309: Find all ways to make a convex shape from the twelve hexiamonds
- 7.2.2.1-320: Find all ways to make a convex shape from the fourteen tetraboloes
- 7.2.2.1-323: Find all ways to make a skewed rectangle from the ten tetraskews
- 7.2.2.1-327: Analyze the Somap graphs
- 7.2.2.1-334: Build fake solutions for Soma-cube shapes
- 7.2.2.1-337: Design a puzzle that makes several kinds of “dice” from the same bent tricubes
- 7.2.2.1-346: Pack space optimally with small tripods
- 7.2.2.1-375: Determine the smallest incomparable dissections of rectangles into rectangles
- 7.2.2.1-387: Classify the types of symmetry that a polycube might have
- 7.2.2.1-394: Prove that every futoshiki puzzle needs at least six clues
- 7.2.2.1-415: Make an exhaustive study of homogenous 5×5 slitherlink
- 7.2.2.1-424: Make an exhaustive study of 6×6 masyu
- 7.2.2.1-432: Find the most interesting 3×3 kakuro puzzles
- 7.2.2.1-442: Enumerate all hitori covers of small grids

Furthermore, I fondly hope that diligent readers will write and say “Dear Don, I have read exercise N and its answer very carefully, and I believe that it is 100% correct,” where N is one of the following exercises in Volume 4 Fascicle 6:

- 7.2.2.2-6: Verify a certain (previously unpublished) lower bound on van der Waerden numbers
*W*(3,*k*) - 7.2.2.2-57: Find a 6-gate way to match a certain 20-variable Boolean function at 32 given points
- 7.2.2.2-165: Devise an algorithm to compute the largest positive autarky of given clauses
- 7.2.2.2-177: Enumerate independent sets of flower snark edges
- 7.2.2.2-212: Prove that partial latin square construction is NP-complete
- 7.2.2.2-282: Find a linear certificate of unsatisfiability for the flower snark clauses
- 7.2.2.2-306-308: Study the reluctant doubling strategy of Luby, Sinclair, and Zuckerman
- 7.2.2.2-318: Find the best possible Local Lemma for
*d*-regular dependency graphs with equal weights - 7.2.2.2-322: Show that random-walk methods cannot always find solutions of locally feasible problems using independent random variables
- 7.2.2.2-335: Express the Möbius series of a cocomparability graph as a determinant
- 7.2.2.2-339: Relate generating functions for traces to generating functions for pyramids
- 7.2.2.2-347: Find the best possible Local Lemma for a given chordal graph with arbitrary weights
- 7.2.2.2-356: Prove the Clique Local Lemma
- 7.2.2.2-363: Study the stable partial assignments of a satisfiability problem
- 7.2.2.2-386: Prove that certain CDCL solvers will efficiently refute any clauses that have a short certificate of unsatisfiability
- 7.2.2.2-428: Show that Boolean functions don't always have forcing representations of polynomial size
- 7.2.2.2-442-444: Study the UC and PC hierarchy of progressively harder sets of clauses
- 7.2.2.2-518: Reduce 3SAT to testing the permanent of a {-1,0,1,2} matrix for zero

Please don't be alarmed by the highly technical nature of these examples;
more than 250 of the *other* exercises are *completely non-scary*,
indeed quite elementary. But of course I do want to go into high-level details also,
for the benefit of advanced readers; and those darker corners of my books
are naturally the most difficult to get right. Hence this plea for help.

Remember that you don't have to work the exercise first. You're allowed
to peek at the answer; in fact, you're even encouraged to do so.
Please send success reports to the usual address for bug reports
(`taocp@cs.stanford.edu`).
Thanks in advance!

By the way, if you want to receive a reward check for discovering an error in TAOCP, your best strategy may well be to scrutinize the answers to the exercises that are listed above.

Meanwhile I continue to work on the final third of Volume 4B, which already has many exciting topics of its own. Those sections are still in very preliminary form, but courageous readers who have nothing better to do might dare to take a peek at the comparatively raw copy in these “prefascicles.” One can look, for instance, at Pre-Fascicle 8a (Hamiltonian Paths and Cycles); Pre-Fascicle 9b (A Potpourri of Puzzles). Thanks to Tom Rokicki, these PostScript files are now searchable!

I seem to get older every day, and people keep asking me to reminisce about the glorious days of yore. If you're interested in checking out some of those videos and other archives, take a look at last year's news page.

Although I must stay home most of the time and work on yet more books that I've promised to complete, I do occasionally get into speaking mode. Nothing is scheduled yet for this year, however, in view of the current pandemic.

Click here for the “recent news” that was current at the end of 2020, if you're interested in old news as well as new news.