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A133922 a(n) = number of permutations (p(1),p(2),p(3),...p(n)) of (1,2,3,...n) such that p(k) is coprime to p(n+1-k) for k = all positive integers <=n. 1
1, 2, 2, 16, 16, 192, 192, 6912, 4608, 230400, 230400, 11612160, 11612160, 1199923200, 588349440, 98594979840, 98594979840, 11076328488960, 11076328488960, 2102897147904000, 1076597725593600, 331238941183180800, 331238941183180800, 66325953940291584000, 56326771107377971200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For n = odd integer the middle term of all counted permutations must be 1.

From Robert Israel, Sep 12 2016:(Start)

Consider the graph with vertices [1,...,n] if n is even, [2,...,n] if n is odd, and edges joining coprime integers.

a(n) is A037223(n) times the number of perfect matchings in this graph.

If n is even, a(n) = A037223(n)*A009679(n/2).

If n is an odd prime, a(n) = a(n-1). (End)

LINKS

Robert Israel, Table of n, a(n) for n = 1..31

EXAMPLE

For n = 6, the permutation (3,2,1,6,4,5) is not counted because p(2)=2 is not coprime to p(5)=4. However, the permutation (3,6,1,4,5,2) is counted because GCD(3,2) = GCD(6,5) = GCD(1,4) = 1.

MAPLE

M:= proc(A) option remember;

    local n, t, i, Ai, Ap, inds, isrt, As;

    n:= nops(A);

    if n = 0 then return 1 fi;

    t:= 0;

    for i in A[1] do

      inds:= [$2..i-1, $i+1..n];

      Ai:= subs([1=NULL, i=NULL, seq(inds[i]=i, i=1..n-2)], A[inds]);

      isrt:= sort([$1..n-2], (r, s) -> nops(Ai(r)) <= nops(Ai(s)), output=permutation);

      Ai:= subs([seq(isrt[i]=i, i=1..n-2)], Ai[isrt]);

      t:= t + procname(Ai);

    od;

    t;

end proc:

F:= proc(n) local A;

  if n::odd then

    if isprime(n) then return procname(n-1) fi;

    A:= [seq(select(t -> igcd(t+1, i+1)=1, [$1..i-1, $i+1..n-1]), i=1..n-1)];

  else

    A:= [seq(select(t -> igcd(t, i)=1, [$1..i-1, $i+1..n]), i=1..n)]

  fi;

  M(A)*floor(n/2)!*2^floor(n/2)

end proc;

seq(F(n), n=1..27); # Robert Israel, Sep 12 2016

CROSSREFS

Cf. A009679, A081123, A037223.

Sequence in context: A093114 A016740 A193145 * A222954 A240033 A088139

Adjacent sequences:  A133919 A133920 A133921 * A133923 A133924 A133925

KEYWORD

hard,nonn

AUTHOR

Leroy Quet, Jan 07 2008

EXTENSIONS

a(6)-a(15) from Sean A. Irvine, May 17 2010

a(16)-a(25) from Robert Israel, Sep 12 2016

STATUS

approved

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Last modified July 21 22:02 EDT 2019. Contains 325210 sequences. (Running on oeis4.)