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Number T(n,k) of permutations p of [n] such that |{ j : |p(j)-j| = 1 }| = k; triangle T(n,k), n >= 0, 0 <= k <= n, read by rows.
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%I #51 Feb 09 2021 08:19:22

%S 1,1,0,1,0,1,2,0,4,0,5,6,10,2,1,21,36,42,12,9,0,117,226,219,104,47,6,

%T 1,792,1568,1472,800,328,64,16,0,6205,12360,11596,6652,2658,688,148,

%U 12,1,55005,109760,103600,60840,24770,7120,1560,200,25,0,543597,1085560,1030649,614420,255830,77732,17750,2876,365,20,1

%N Number T(n,k) of permutations p of [n] such that |{ j : |p(j)-j| = 1 }| = k; triangle T(n,k), n >= 0, 0 <= k <= n, read by rows.

%H Alois P. Heinz, <a href="/A320582/b320582.txt">Rows n = 0..24, flattened</a>

%F Sum_{k=1..n} k * T(n,k) = A052582(n-1) for n > 0.

%F Sum_{k=0..n} (k+1) * T(n,k) = A082033(n-1) for n > 0.

%e T(4,0) = 5: 1234, 1432, 3214, 3412, 4231.

%e T(4,1) = 6: 2431, 3241, 3421, 4132, 4213, 4312.

%e T(4,2) = 10: 1243, 1324, 1342, 1423, 2134, 2314, 2413, 3124, 3142, 4321.

%e T(4,3) = 2: 2341, 4123.

%e T(4,4) = 1: 2143.

%e Triangle T(n,k) begins:

%e 1;

%e 1, 0;

%e 1, 0, 1;

%e 2, 0, 4, 0;

%e 5, 6, 10, 2, 1;

%e 21, 36, 42, 12, 9, 0;

%e 117, 226, 219, 104, 47, 6, 1;

%e 792, 1568, 1472, 800, 328, 64, 16, 0;

%e 6205, 12360, 11596, 6652, 2658, 688, 148, 12, 1;

%e 55005, 109760, 103600, 60840, 24770, 7120, 1560, 200, 25, 0;

%e ...

%p b:= proc(s) option remember; expand((n-> `if`(n=0, 1, add(

%p `if`(abs(n-j)=1, x, 1)*b(s minus {j}), j=s)))(nops(s)))

%p end:

%p T:= n-> (p-> seq(coeff(p, x, i), i=0..n))(b({$1..n})):

%p seq(T(n), n=0..12);

%t b[s_] := b[s] = Expand[With[{n = Length[s]}, If[n==0, 1, Sum[

%t If[Abs[n-j]==1, x, 1]*b[s~Complement~{j}], {j, s}]]]];

%t T[n_] := PadRight[CoefficientList[b[Range[n]], x], n+1];

%t T /@ Range[0, 12] // Flatten (* _Jean-François Alcover_, Feb 09 2021, after _Alois P. Heinz_ *)

%Y Column k=0 gives A078480.

%Y Row sums give A000142.

%Y Main diagonal gives A059841.

%Y Cf. A008290, A008291, A052582, A082033, A323671.

%K nonn,tabl

%O 0,7

%A _Alois P. Heinz_, Jan 23 2019