

A174702


The number of permutations p of {1,...,n} such that p(i)p(i+1) is in {1,2,3,4,5} for all i from 1 to n1.


12



1, 2, 6, 24, 120, 720, 3600, 15600, 61872, 236388, 901748, 3509106, 13716168, 53327912, 205176192, 780194112, 2937412512, 10991746368, 40961976672, 152144989056
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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 {1,2,3,4,5}.


LINKS

Table of n, a(n) for n=1..20.
W. Edwin Clark, permutations p in S_n such that m <= p(i)p(i+1) <= M for i from 1 to n1, SeqFan Discussion, Mar 2010.


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, 5, n): seq (a(n), n=1..10); # Alois P. Heinz, Mar 27 2010


CROSSREFS

Cf. A003274, A174700, A174701, A174703, A174704, A174705, A174706, A174707, A174708, A185030, A216837.
Sequence in context: A189853 A189857 A189860 * A173846 A154654 A189862
Adjacent sequences: A174699 A174700 A174701 * A174703 A174704 A174705


KEYWORD

more,nonn


AUTHOR

W. Edwin Clark, Mar 27 2010


EXTENSIONS

a(15)a(20) from R. H. Hardin, May 06 2010


STATUS

approved



