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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 n-1. 13
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 (list; graph; refs; listen; history; text; internal format)
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 |i-j| 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[t-1])); 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[t-1])); if m<=d and d<=M then p(t+1) fi od; h:= l[t]; for j from t to n-1 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

MATHEMATICA

f[m_, M_, n_] := f[m, M, n] = Module[{i, l, p, cnt}, Do[l[i] = i, {i, 1, n}]; cnt = 0; p[t_] := Module[{d, j, h}, If[t == n, d = If[t == 1, m, Abs[l[t] - l[t-1]]]; If [m <= d && d <= M, cnt = cnt+1], For[j = t, j <= n, j++, {l[t], l[j]} = {l[j], l[t]}; d = If[t == 1, m, Abs[l[t] - l[t-1]]]; If [m <= d && d <= M, p[t+1]]]; h = l[t]; For[j = t, j <= n-1, j++, l[j] = l[j+1]]; l[n] = h]]; p[1]; cnt]; a[n_] := f[1, 3, n]; Table[Print["a(", n, ") = ", a[n]]; a[n], {n, 1, 15}] (* slow beyond n = 15 *) (* Jean-Fran├žois Alcover, Jun 01 2015, after Alois P. Heinz *)

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

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Last modified August 29 10:37 EDT 2015. Contains 261188 sequences.