

A174700


The number of permutations p of {1,...,n} such that p(i)p(i+1) is in {1,2,3} for all i from 1 to n1.


12



1, 2, 6, 24, 72, 180, 428, 1042, 2512, 5912, 13592, 30872, 69560, 155568, 345282, 761312, 1669612, 3645236, 7927404, 17180092, 37119040, 79986902, 171964534, 368959906, 790214816, 1689779842, 3608413750, 7696189046
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OFFSET

1,2


COMMENTS

For n>1, a(n)/2 is the number of Hamiltonian paths on the graph with vertex set {1,...,n} where i is adjacent to j iff ij is in {1,2,3}.


LINKS

Table of n, a(n) for n=1..28.


MAPLE

f:= proc(m, M, n) option remember; local i, l, p, cnt; l:= array([i$i=1..n]); cnt:=0; p:= proc(t) local d, j, h; if t=n then d:=`if`(t=1, m, abs(l[t]l[t1])); if m<=d and d<=M then cnt:= cnt+1 fi else for j from t to n do l[t], l[j]:= l[j], l[t]; d:=`if`(t=1, m, abs(l[t]l[t1])); if m<=d and d<=M then p(t+1) fi od; h:= l[t]; for j from t to n1 do l[j]:= l[j+1] od; l[n]:= h fi end; p(1); cnt end: a:=n>f(1, 3, n); # Alois P. Heinz, Mar 27 2010


CROSSREFS

Cf. A003274, A174701, A174702, A174703, A174704, A174705, A174706, A174707, A174708, A185030, A216837.
Sequence in context: A236625 A096259 A087645 * A216158 A178847 A173844
Adjacent sequences: A174697 A174698 A174699 * A174701 A174702 A174703


KEYWORD

nonn


AUTHOR

W. Edwin Clark, Mar 27 2010


EXTENSIONS

a(19)a(28) from R. H. Hardin, May 06 2010


STATUS

approved



