

A047992


Number of distinct permutations generated by shuffling n cards with "clump size" <= 2.


3



2, 5, 10, 16, 26, 42, 68, 110, 178, 288, 466, 754, 1220, 1974, 3194
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OFFSET

2,1


COMMENTS

Take a deck of n cards, cut into two nonempty piles, then do a riffleshuffle in which no more than 2 consecutive cards fall from the same half. Sequence gives number of distinct npermutations that result.


LINKS

Table of n, a(n) for n=2..16.


FORMULA

For n>3, a(n) = 2 * F(n+1), with F(n) = A000045(n).


EXAMPLE

a(4)=10 because we can split the deck as 1234 then shuffle to get 2134 or 2314, or split as 1234 and get 3421 1324 1342 3124 3142 or split 1234 and get 1243, 1423. These plus the identity (1234) give 10 permutations in all.


CROSSREFS

Essentially the same as A006355.
Sequence in context: A011903 A078435 A049815 * A079984 A027613 A192701
Adjacent sequences: A047989 A047990 A047991 * A047993 A047994 A047995


KEYWORD

nonn


AUTHOR

Mike Keith (domnei(AT)aol.com)


STATUS

approved



