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A005263
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Number of labeled Greg trees.
(Formerly M3647)
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8
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1, 1, 1, 4, 32, 396, 6692, 143816, 3756104, 115553024, 4093236352, 164098040448, 7345463787136, 363154251536896, 19653476190481408, 1155636468524067328, 73364615077878838784, 5001199614295920565248, 364363128390631094137856
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,4
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COMMENTS
| A Greg tree can be described as a tree with 2-colored nodes where only the black nodes are counted and labeled and the white nodes are of degree at least 3.
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REFERENCES
| C. Flight, How many stemmata?, Manuscripta, 34 (1990), 122-128.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
| Index entries for sequences related to trees
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FORMULA
| E.g.f.: 1+B(x)-B(x)^2 where B(x) is e.g.f. of A005264.
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PROG
| (PARI) {a(n)= local(A); if(n<1, n==0, for(k= 1, n, A+= x*O(x^k); A= truncate( (1+x)* exp(A) -1-A) ); A+= x*O(x^n); A-= A^2; n!* polcoeff( A, n))} /* Michael Somos Apr 02 2007 */
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CROSSREFS
| Cf. A005264, A005640, A048159, A048160, A052300-A052303.
Sequence in context: A007763 A195193 A203435 * A113131 A195762 A127670
Adjacent sequences: A005260 A005261 A005262 * A005264 A005265 A005266
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KEYWORD
| nonn,nice,easy
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
| More terms, formula and comment from Christian G. Bower (bowerc(AT)usa.net), Nov 15 1999.
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