login
This site is supported by donations to The OEIS Foundation.
Logo

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A109457 Number of Krom functions on n variables (or 2SAT instances): conjunctions of clauses with two literals per clause. 4
2, 4, 16, 166, 4170, 224716, 24445368, 5167757614, 2061662323954 (list; graph; refs; listen; history; internal format)
OFFSET

0,1

COMMENTS

A Krom function is equivalent to a Boolean function with the property that, if f(x)=f(y)=f(z)=1, then f(<xyz>)=1, where <xyz> denotes the bitwise median of the three Boolean vectors x, y, z.

Also related to number of retracts of an n-cube (see Feder).

REFERENCES

Tomas Feder, Stable Networks and Product Graphs, Memoirs of the American Mathematical Society, 555 (1995), Section 3.2.

D. E. Knuth, The Art of Computer Programming, Vol. 4A, Section 7.1.1, p. 79.

M. R. Krom, The decision problem for a class of first-order formulas in which all disjunctions are binary, Zeitschrift f. mathematische Logik und Grundlagen der Mathematik, 13 (1967), 15-20.

Thomas J. Schaefer, The complexity of satisfiability problems, ACM Symposium on Theory of Computing, 10 (1978), 216-226.

CROSSREFS

Cf. A109458, A109459, A102897.

Cf. A112535.

Sequence in context: A061588 A202360 A050472 * A105788 A071008 A178077

Adjacent sequences:  A109454 A109455 A109456 * A109458 A109459 A109460

KEYWORD

nonn,hard

AUTHOR

D. E. Knuth, Aug 24 2005

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Puzzles | Hot | Classics
Recent Additions | More pages | Superseeker | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified February 13 16:02 EST 2012. Contains 205521 sequences.