

A155971


Genlex listings of (s,t)combination strings possessing the revolvingdoor property.


0



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
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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 weightt strings from a given starting point to a given ending point via revolvingdoor moves." (See reference Knuth.)


REFERENCES

D. E. Knuth, The Art of Computer Programming, Volume 4, Fascicle 3, Generating all Combinations and Partitions, AddisonWesley 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



