A089973 Number of maximum-length 2-surprising sequences in n symbols.

1, 6, 24, 48, 25440, 554400

Offset

1,2

Comments

A sequence of symbols is 2-surprising if, for every pair of symbols X and Y, not necessarily distinct and every distance D, there is at most one position in t he sequence where X precedes Y by distance D. The maximum lengths are given in A008062.

References

Dennis E. Shasha, Puzzling Adventures, Scientific American 289(#12, 2003), 22.

Links

Table of n, a(n) for n=1..6.

Example

The 2-surprising sequences in 3 symbols are:
(2012100) (1021200) (1120210) (2011210) (1202210) (2210120) (2101120) (1022120)
(2100201) (0021201) (0212201) (2102011) (0120211) (2010021) (2201021) (0122021)
(1200102) (0121102) (0012102) (1020012) (0211012) (1102012) (1201022) (0210122)
Thus a(3)=24.

Crossrefs

Cf. A008062.

Sequence in context: A000056 A083170 A087081 * A277014 A033581 A213393

Adjacent sequences: A089970 A089971 A089972 * A089974 A089975 A089976

Keyword

nonn

Author

John W. Layman, Nov 17 2003

Status

approved