The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A058876 Triangle read by rows: T(n,k) = number of labeled acyclic digraphs with n nodes, containing exactly n+1-k points of in-degree zero (n >= 1, 1<=k<=n). 5
 1, 1, 2, 1, 9, 15, 1, 28, 198, 316, 1, 75, 1610, 10710, 16885, 1, 186, 10575, 211820, 1384335, 2174586, 1, 441, 61845, 3268125, 64144675, 416990763, 654313415, 1, 1016, 336924, 43832264, 2266772550, 44218682312, 286992935964, 450179768312 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 REFERENCES F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 19, (1.6.4). R. W. Robinson, Counting labeled acyclic digraphs, pp. 239-273 of F. Harary, editor, New Directions in the Theory of Graphs. Academic Press, NY, 1973. LINKS R. W. Robinson, Enumeration of acyclic digraphs, Manuscript. (Annotated scanned copy) FORMULA Harary and Prins (following Robinson) give a recurrence. EXAMPLE Triangle begins: 1; 1,  2; 1,  9,  15; 1, 28, 198, 316; ... MATHEMATICA a[p_, k_] :=a[p, k] =If[p == k, 1, Sum[Binomial[p, k]*a[p - k, n]*(2^k - 1)^n*2^(k (p - k - n)), {n, 1, p - k}]]; Map[Reverse, Table[Table[a[p, k], {k, 1, p}], {p, 1, 6}]] // Grid (* Geoffrey Critzer, Aug 29 2016 *) CROSSREFS Columns give A058877, A003025, A003026. Row sums give A003024. Sequence in context: A180001 A204371 A199887 * A214884 A083162 A178075 Adjacent sequences:  A058873 A058874 A058875 * A058877 A058878 A058879 KEYWORD nonn,easy,tabl AUTHOR N. J. A. Sloane, Jan 07 2001 EXTENSIONS More terms from Vladeta Jovovic, Apr 10 2001 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 23 20:42 EDT 2021. Contains 347617 sequences. (Running on oeis4.)