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 A256061 Number T(n,k) of 2n-length strings of balanced parentheses of exactly k different types; triangle T(n,k), n>=0, 0<=k<=n, read by rows. 6
 1, 0, 1, 0, 2, 4, 0, 5, 30, 30, 0, 14, 196, 504, 336, 0, 42, 1260, 6300, 10080, 5040, 0, 132, 8184, 71280, 205920, 237600, 95040, 0, 429, 54054, 774774, 3603600, 7207200, 6486480, 2162160, 0, 1430, 363220, 8288280, 58378320, 180180000, 273873600, 201801600, 57657600 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS Also number of binary trees with n inner nodes of exactly k different dimensions.  T(2,2) = 4: : balanced parentheses :  ([]) :  [()] : ()[]  : []()  : :----------------------:-------:-------:-------:-------: :                trees :   (1) :   [2] : (1)   : [2]   : :                      :   / \ :   / \ : / \   : / \   : :                      : [2]   : (1)   :   [2] :   (1) : :                      : / \   : / \   :   / \ :   / \ : LINKS Alois P. Heinz, Rows n = 0..140, flattened FORMULA T(n,k) = Sum_{i=0..k} (-1)^i * C(k,i) * (k-i)^n * A000108(n). T(n,k) = k! * A253180(n,k). T(n,k) = Sum_{i=0..k} (-1)^i * C(k,i) * A290605(n,k-i). - Alois P. Heinz, Oct 28 2019 EXAMPLE A(3,2) = 30: (())[], (()[]), (([])), ()()[], ()([]), ()[()], ()[[]], ()[](), ()[][], ([()]), ([[]]), ([]()), ([])(), ([])[], ([][]), [(())], [()()], [()[]], [()](), [()][], [([])], [[()]], [[]()], [[]](), [](()), []()(), []()[], []([]), [][()], [][](). Triangle T(n,k) begins:   1;   0,   1;   0,   2,     4;   0,   5,    30,     30;   0,  14,   196,    504,     336;   0,  42,  1260,   6300,   10080,    5040;   0, 132,  8184,  71280,  205920,  237600,   95040;   0, 429, 54054, 774774, 3603600, 7207200, 6486480, 2162160; MAPLE ctln:= proc(n) option remember; binomial(2*n, n)/(n+1) end: A:= proc(n, k) option remember; k^n*ctln(n) end: T:= (n, k)-> add(A(n, k-i)*(-1)^i*binomial(k, i), i=0..k): seq(seq(T(n, k), k=0..n), n=0..10); MATHEMATICA A[0, 0] = 1; A[n_, k_] := A[n, k] = k^n*CatalanNumber[n]; T[n_, k_] := Sum[A[n, k-i]*(-1)^i*Binomial[k, i], {i, 0, k}]; Table[T[n, k], {n, 0, 10}, {k, 0, n}] // Flatten (* Jean-François Alcover, Feb 20 2017, translated from Maple *) CROSSREFS Columns k=0-1 give: A000007, A000108 (for n>0). Main diagonal gives A001761. Cf. A253180, A255982, A258427, A290605. Sequence in context: A269011 A274086 A255982 * A323099 A002652 A202541 Adjacent sequences:  A256058 A256059 A256060 * A256062 A256063 A256064 KEYWORD nonn,tabl AUTHOR Alois P. Heinz, Mar 13 2015 STATUS approved

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Last modified January 18 09:26 EST 2022. Contains 350454 sequences. (Running on oeis4.)