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A227787
The number of n X n tomographically balanced matrices.
0
1, 1, 1, 3, 31, 877, 107637, 66291089, 214925869109
OFFSET
1,4
COMMENTS
A tomographically balanced matrix is a matrix whose entries are -1, 0, or +1, with the property that the sum over any row, any column, or any of the diagonal lines with slope plus-or-minus 1 is zero.
REFERENCES
Donald E. Knuth, Satisfiability, Fascicle 6, volume 4 of The Art of Computer Programming, Addison-Wesley, 2015, pages 141 and 204-205, Exercise 105.
EXAMPLE
For n=4 the three cases are 0000/0000/0000/0000, 0+-0/-00+/+00-/0-+0, and 0-+0/+00-/-00+/0+-0.
CROSSREFS
Sequence in context: A317348 A144416 A362846 * A356673 A274592 A219266
KEYWORD
nonn,hard,more
AUTHOR
Don Knuth, Sep 23 2013
EXTENSIONS
a(9) from Bert Dobbelaere, Sep 14 2020
STATUS
approved