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 A225942 Triangular array read by rows: T(n,k) is the number of f:{1,2,...,n}->{1,2,...,n} with exactly 2k elements that have a preimage of even (possibly zero) cardinality; n>=0, 0<=k<=floor(n/2). 1
 1, 1, 2, 2, 6, 21, 24, 192, 40, 120, 1800, 1205, 720, 18000, 25680, 2256, 5040, 194040, 489510, 134953, 40320, 2257920, 9031680, 5196800, 250496, 362880, 28304640, 167015520, 166793760, 24943689, 3628800, 381024000, 3149798400, 4904524800, 1514960640, 46063360 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Urn A is initially filled with n labeled balls while urn B is empty. A ball is randomly selected and switched from one urn to the other. T(n,k)/n^n is the probability that urn A contains 2k balls after n switches have been made. Row sums = n^n. T(n,0)   = n!. T(2n,n)  = A209289(n). LINKS Alois P. Heinz, Rows n = 0..100, flattened FORMULA T(n,k) = n! * [x^n*y^(2k)] (y*cosh(x)+sinh(x))^n. EXAMPLE 1; 1; 2,    2; 6,    21; 24,   192,    40; 120,  1800,   1205; 720,  18000,  25680,  2256; 5040, 194040, 489510, 134953; MATHEMATICA Map[Select[#, # > 0 &] &, Prepend[Table[nn = n;     CoefficientList[      Expand[n! Coefficient[         Series[(y Cosh[x] + Sinh[x])^n, {x, 0, nn}], x^n]], y], {n, 1,       7}], {1}]] // Grid CROSSREFS Sequence in context: A171694 A103179 A320932 * A004077 A271217 A202743 Adjacent sequences:  A225939 A225940 A225941 * A225943 A225944 A225945 KEYWORD nonn,tabf AUTHOR Geoffrey Critzer, May 21 2013 STATUS approved

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Last modified May 9 19:47 EDT 2021. Contains 343746 sequences. (Running on oeis4.)