A155971 Genlex listings of (s,t)-combination strings possessing the revolving-door property.

1, 1, 1, 1, 2, 1, 1, 4, 4, 1, 1, 8, 20, 8, 1, 1, 16, 160, 160, 16, 1, 1, 32, 2264, 17152, 2264, 32, 1

Offset

0,5

Comments

D. E. Knuth writes: "No simple formula seems to exist, but the listings can be counted for small s and t by systematically computing the numbers of genlex paths that run through all weight-t strings from a given starting point to a given ending point via revolving-door moves." (See reference Knuth.)

References

D. E. Knuth, The Art of Computer Programming, Volume 4, Fascicle 3, Generating all Combinations and Partitions, Addison-Wesley 2005, section 7.2.1.3, solution to exercise 32.

Links

Table of n, a(n) for n=0..27.

Example

1
1, 1
1, 2, 1
1, 4, 4, 1
1, 8, 20, 8, 1
1, 16, 160, 160, 16, 1
1, 32, 2264, 17152, 2264, 32, 1

Crossrefs

Sequence in context: A268056 A268079 A202549 * A176480 A154218 A326326

Adjacent sequences: A155968 A155969 A155970 * A155972 A155973 A155974

Keyword

nonn,more,tabl

Author

Peter Luschny, Jan 31 2009

Status

approved