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A007016 Number of permutations of length n with 1 fixed and 1 reflected point.
(Formerly M4491)
25
0, 1, 0, 0, 8, 20, 96, 656, 5568, 48912, 494080, 5383552, 65097600, 840566080, 11833898496, 176621049600, 2838024476672, 48060623405312, 868000333234176, 16441638519762944, 329723762151352320, 6907027877807330304 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

Number of distinct solutions to the order n checkerboard problem, including symmetrical solutions: place n pieces on an n X n board so there is exactly one piece in each row, column and main diagonal. Compare A064280.

Number of magic permutation matrices of order n. - Chai Wah Wu, Jan 15 2019

REFERENCES

Simpson, Todd; Permutations with unique fixed and reflected points. Ars Combin. 39 (1995), 97-108.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Table of n, a(n) for n=0..21.

T. Simpson, Letter to N. J. A. Sloane, Mar. 1992

T. Simpson, Permutations with unique fixed and reflected points, Preprint. (Annotated scanned copy)

F. Rakotondrajao, Magic squares, rook polynomials and permutations, Seminaire Lotharingien de Combinatoire, vol. 54A, article B54Ac, 2006.

FORMULA

a(2*m) = m*(x(2*m) - (2*m-3)*x(2*m-1)), a(2*m+1) = (2*m+1)*x(2*m) + 3*m*x(2*m-1) - 2*m*(m-1)*x(2*m-2), where x(n) = A003471(n).

MATHEMATICA

x[n_] := x[n] = Integrate[If[EvenQ[n], (x^2 - 4*x + 2)^(n/2), (x - 1)*(x^2 - 4*x + 2)^((n - 1)/2)]/E^x, {x, 0, Infinity}];

a[n_ /; EvenQ[n]] := With[{m = n/2}, m*(x[2*m] - (2*m - 3)*x[2*m - 1])];

a[n_ /; OddQ[n]] := With[{m = (n - 1)/2}, (2*m + 1)*x[2*m] + 3*m*x[2*m - 1] - 2*m*(m - 1)*x[2*m - 2]];

Table[a[n], {n, 0, 21}] // Quiet (* Jean-Fran├žois Alcover, Jun 29 2018 *)

PROG

(PARI)

a(n) = {my(v = vector(n)); \\ v is A003471

for(n=4, length(v), v[n] = (n-1)*v[n-1] + 2*if(n%2==1, (n-1)*v[n-2], (n-2) * if(n==4, 1, v[n-4])));

if(n<4, [1, 0, 0][n], if(n%2==0, n*(v[n] - (n-3)*v[n-1]), 2*n*v[n-1] + 3*(n-1)*v[n-2] - (n-1)*(n-3)*v[n-3])/2)} \\ Andrew Howroyd, Sep 12 2017

CROSSREFS

Cf. A003471, A064280.

Sequence in context: A101363 A003685 A066011 * A129550 A215181 A014584

Adjacent sequences:  A007013 A007014 A007015 * A007017 A007018 A007019

KEYWORD

nonn,easy,changed

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified January 17 05:26 EST 2019. Contains 319207 sequences. (Running on oeis4.)