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 A283184 a(n) is the number of symmetric permutations (p(1),p(2),...,p(m)) of (1,2,...,m), m=2n or m=2n+1, with p(m+1-k) = m+1-p(k) for 1<=k<=m, such that adjacent numbers do not differ by 1. a(n) is also the number of point-symmetric arrangements of m non-attacking kings on an m X m board, with one in each row and column. 1
 1, 0, 2, 14, 122, 1262, 15466, 219646, 3551194, 64431374, 1296712778, 28672204574, 691007296954, 18029138380846, 506320912190506, 15228632768870462, 488405396197019546, 16638380026019579726, 600022595692147574794, 22836184629309211495774, 914717041435012519583098 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS For m=2n+1 the symmetry requires p(n+1)=n+1. That is why the number of permutations is the same for m=2n and m=2n+1. The n-th element of any permutation is not allowed to be n because otherwise the next element would be n+1. Because of the symmetry it is sufficient to consider the first n elements. Any such n-tuple can be created by a permutation of length n, last element smaller than n: Each element b(k) > n has to be replaced by m+1-b(k). Example m=6: Original symmetric permutation 536142, 3-tuple 536 created by 231: 5 is replaced by 7-5 and 6 by 7-6. How many such n-tuples can be created by a n-permutation? Let us analyze the example above: There are two pairs of adjacent numbers (23 and 31) in the permutation 231. The difference of the first pair is 1, so either 2 or 3 must be replaced, whereas the second pair represents a "gap" (difference > 1), so that 1 can be kept or replaced by 6. This way, 231 creates four 3-tuples: 531, 241, 536, 246. Let generally g be the number of gaps in a n-permutation (0<=g<=n-1). Then the number of related n-tuples is 2^(g+1) because the first element of the permutation and each element behind a gap can be arbitrarily replaced or not. On the other hand, when the first element of a section between successive gaps is selected, there is no choice for the replacement of the other elements. When q is a n-permutation, the number of gaps is g(q) = Sum_{j=1..n-1} sign(|p(j+1)-p(j)|-1). (sign = signum) The extension up to n=50 was done by a new algorithm, see link "Fast recurrence". - Gerhard Kirchner, Mar 17 2017 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..400 (terms n = 0..50 from Gerhard Kirchner) Gerhard Kirchner, Fast recurrence FORMULA Let q be any permutation (p(1), p(2),..., p(n)) with p(n) < n and g(q) = Sum_{j=1..n-1} sgn(|p(j+1)-p(j)|-1). a(n) = Sum_{each q} 2^(g(q)+1). a(n) ~ exp(-1) * 2^n * n!. - Vaclav Kotesovec, Apr 20 2017 EXAMPLE Example 1, m=5: The matrix, transforming 12345 into 41352, can also be thought of as a chessboard; each "1" is a king. ./0 0 0 1 0\  /1\ /4\ | 1 0 0 0 0 | |2| |1| | 0 0 1 0 0 |*|3|=|3| | 0 0 0 0 1 | |4| |5| .\0 1 0 0 0/  \5/ \2/ Example 2, m=6: q is a 3-permutation not ending on 3: q   g(q)  2^(g(q)+1) Symmetric 6-permutations, |p(j+1)-p(j)|>1 132   1        4        135246, 635241, 142536, 642531 231   1        4        531642, 536142, 241635, 246135 312   1        4        315264, 362514, 462513, 415263 321   0        2        426153, 351624 Result: a(3)=14. MAPLE b:= proc(n, s, l) option remember; `if`(s={},      `if`(abs(n/2-l)<1, 0, 1), add(add(`if`(abs(j-l)=1, 0,         b(n, s minus {i}, i)), j=[i, n-i]), i=s))     end: a:= n-> b(2*n+1, {\$1..n}, -1): seq(a(n), n=0..10);  # Alois P. Heinz, Mar 15 2017 # second Maple program: a:= proc(n) option remember; `if`(n<4, (n-1)*       (7*n^2-5*n-6)/6, (2*n+1)*a(n-1) -(2*n-5)*       (a(n-2)+a(n-3)) +(2*n-6)*a(n-4))     end: seq(a(n), n=0..20);  # Alois P. Heinz, Mar 17 2017 MATHEMATICA a[n_] := a[n] = If[n<4, (n-1)*(7n^2-5n-6)/6, (2n+1)*a[n-1] - (2n-5)*(a[n-2] + a[n-3]) + (2n-6)*a[n-4]]; Table[a[n], {n, 0, 20}] (* Jean-François Alcover, Mar 18 2017, after Alois P. Heinz *) PROG (VB) a(n) = fusum(n, 1, "", "", 0, 0) with Function fusum(n, t, permu\$, pile\$, g, su) If t = n + 1 Then   su = su + 2 ^ (g + 1) Else   la = n + 1 - t   If t = 1 Then     la = n - 1     For k = 1 To n: pile\$ = pile\$ + Chr(k): Next   End If   For s = la To 1 Step -1     y = Asc(Mid(pile\$, s))     If t = 1 Then ad = 0 Else ad = Sgn(Abs(y - Asc(permu\$)) - 1)     fusum = fusum(n, t + 1, Chr(y) + permu\$, Left(pile\$, s - 1) + Mid(pile\$, s + 1), g + ad, su)   Next   If t = n Then fusum = su End If End Function CROSSREFS Cf. A002464. Sequence in context: A155728 A267906 A199560 * A319536 A060468 A349261 Adjacent sequences:  A283181 A283182 A283183 * A283185 A283186 A283187 KEYWORD nonn AUTHOR Gerhard Kirchner, Mar 02 2017 EXTENSIONS a(14)-a(20) from Alois P. Heinz, Mar 15 2017 STATUS approved

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Last modified October 1 02:38 EDT 2022. Contains 357134 sequences. (Running on oeis4.)