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 A219859 Triangular array read by rows: T(n,k) is the number of endofunctions, functions f:{1,2,...,n}->{1,2,...,n}, that have exactly k elements with no preimage; n>=0, 0<=k<=n. 1
 1, 1, 0, 2, 2, 0, 6, 18, 3, 0, 24, 144, 84, 4, 0, 120, 1200, 1500, 300, 5, 0, 720, 10800, 23400, 10800, 930, 6, 0, 5040, 105840, 352800, 294000, 63210, 2646, 7, 0, 40320, 1128960, 5362560, 7056000, 2857680, 324576, 7112, 8, 0, 362880, 13063680, 83825280, 160030080, 105099120, 23496480, 1524600, 18360, 9, 0 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Equivalently, T(n,k) is the number of endofunctions whose functional digraph has exactly k leaves. Equivalently, T(n,k) is the number of doubly rooted trees with k leaves.  Here, a doubly rooted tree is a labeled tree in which two special vertices have been selected and the order of the selection matters. [Bona page 266] Row sums are n^n. REFERENCES M. Bona, Introduction to Enumerative Combinatorics, McGraw Hill, 2007. LINKS FORMULA T(n,k) = n!/k! * Stirling2(n,n-k). T(n,0) = n!. T(n,k) = A055302(n,k)*(n-k) + A055302(n,k+1)*(k+1).  The first term (on rhs of this equation) is the number of such functions in which the preimage of f(n) contains more than one element. The second term is the number of such functions in which the preimage of f(n) contains exactly one element. T(n,k) = binomial(n,k) Sum_{j=0..n-k}(-1)^j*binomial(n-k,j)*(n-k-j)^n. - Geoffrey Critzer, Aug 20 2013 EXAMPLE 1; 1,   0; 2,   2,     0; 6,   18,    3,     0; 24,  144,   84,    4,     0; 120, 1200,  1500,  300,   5,   0; 720, 10800, 23400, 10800, 930, 6,  0; MATHEMATICA Table[Table[n!/k!StirlingS2[n, n-k], {k, 0, n}], {n, 0, 8}]//Grid CROSSREFS Cf. A055314. Sequence in context: A323777 A292317 A285675 * A168615 A174104 A296492 Adjacent sequences:  A219856 A219857 A219858 * A219860 A219861 A219862 KEYWORD nonn,tabl AUTHOR Geoffrey Critzer, Dec 01 2012 STATUS approved

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Last modified February 26 04:23 EST 2020. Contains 332275 sequences. (Running on oeis4.)