|
|
A076220
|
|
Number of permutations of 1..n in which every pair of adjacent numbers are relatively prime.
|
|
15
|
|
|
1, 1, 2, 6, 12, 72, 72, 864, 1728, 13824, 22032, 555264, 476928, 17625600, 29599488, 321115392, 805146624, 46097049600, 36481536000, 2754120268800, 3661604352000, 83905105305600, 192859121664000, 20092043520000000, 15074060547686400, 1342354557616128000
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,3
|
|
LINKS
|
|
|
FORMULA
|
|
|
EXAMPLE
|
a(4) = 12 since there are 12 permutations of 1234 in which every 2 adjacent numbers are relatively prime: 1234, 1432, 2134, 2143, 2314, 2341, 3214, 3412, 4123, 4132, 4312, 4321.
|
|
MAPLE
|
with(combinat): for n from 1 to 7 do P:=permute(n): ct:=0: for j from 1 to n! do if add(gcd(P[j][i+1], P[j][i]), i=1..n-1)=n-1 then ct:=ct+1 else ct:=ct fi od: a[n]:=ct: od: seq(a[n], n=1..7); # Emeric Deutsch, Mar 28 2005
# second Maple program:
b:= proc(s, t) option remember; `if`(s={}, 1, add(
`if`(igcd(i, t)>1, 0, b(s minus {i}, i)), i=s))
end:
a:= n-> b({$1..n}, 1009):
|
|
MATHEMATICA
|
f[n_] := Block[{p = Permutations[ Table[i, {i, 1, n}]], c = 0, k = 1}, While[k < n! + 1, If[ Union[ GCD @@@ Partition[p[[k]], 2, 1]] == {1}, c++ ]; k++ ]; c]; Do[ Print[ f[n]], {n, 2, 15}]
|
|
PROG
|
(PARI) {A076220(n)=local(A, d, n, r, M); A=matrix(n, n, i, j, if(gcd(i, j)==1, 1, 0)); r=0; for(s=1, 2^n-1, M=vecextract(A, s, s)^(n-1); d=matsize(M)[1]; r+=(-1)^(n-d)*sum(i=1, d, sum(j=1, d, M[i, j]))); r} \\ Max Alekseyev, Jun 12 2005
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|