

A095816


Number of permutations of 1..n with no three elements in correct or reverse order.


8



1, 1, 2, 4, 18, 92, 570, 4082, 33292, 304490, 3086890, 34357812, 416526730, 5463479106, 77094352076, 1164544912938, 18749754351338, 320544941916628, 5799226664694602, 110695180631374114, 2223242026407894732
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OFFSET

0,3


COMMENTS

Counts permutations with the property that no subsequence i(i+1)(i+2) or (i+2)(i+1)i occurs.


LINKS

Table of n, a(n) for n=0..20.
Jackson, D. M. and Read, R. C., A note on permutations without runs of given length, Aequationes Math. 17 (1978), no. 23, 336343.


FORMULA

G.f. for number of permutations of 1..n with no m elements in correct or reverse order is Sum(n!*((2*x^mx^(m+1)x)/(x^m1))^n,n=0..infinity).  Ivana Jovovic ( ivana121(AT)EUnet.yu ), Nov 11 2007


CROSSREFS

Cf. A002464, A095817, A095818.
Cf. A165963, A165964, A078628. [From Isaac Lambert, Oct 07 2009]
Sequence in context: A014448 A075836 A120664 * A020101 A099938 A135069
Adjacent sequences: A095813 A095814 A095815 * A095817 A095818 A095819


KEYWORD

nonn


AUTHOR

Jonas Wallgren, Jun 08 2004


EXTENSIONS

More terms from Ivana Jovovic ( ivana121(AT)EUnet.yu ), Nov 11 2007
a(0)=1 prepended by Max Alekseyev, Jun 14 2011


STATUS

approved



