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A013587
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Number of rational plane curves of degree d passing through 3d-1 general points.
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8
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1, 1, 12, 620, 87304, 26312976, 14616808192, 13525751027392, 19385778269260800, 40739017561997799680, 120278021410937387514880, 482113680618029292368686080, 2551154673732472157928033617920, 17410560213476464590484763013222400
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OFFSET
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1,3
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REFERENCES
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M. Atiyah, On the unreasonable effectiveness of physics in mathematics, in "Highlights of Mathematical Physics", ed. A. S. Fokas, pp. 25ff.
D. A. Cox and S. Katz, Mirror Symmetry and Algebraic Geometry, American Mathematical Society, 1999, p. 198.
P. DiFranceso and C. Itzykson, Quantum intersection rings, in The Moduli Space of Curves, Birkhäuser, Boston, 1995, pp. 81-148.
W. Fulton, Enumerative geometry via quantum cohomology, lecture notes, AMS Summer Institute, Santa Cruz, 1995.
Yuri I. Manin, Frobenius Manifolds, Quantum Cohomology and Moduli Spaces, American Mathematical Society, 1999, p. 7.
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LINKS
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Steven R. Finch, Enumerative geometry, February 24, 2014. [Cached copy, with permission of the author]
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FORMULA
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a_d = Sum_{i+j=d} a_i*a_j ( i^2*j^2*binomial(3d-4, 3i-2) - i^3*j*binomial(3d-4, 3i-1) ).
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EXAMPLE
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G.f. = x + x^2 + 12*x^3 + 620*x^4 + 87304*x^5 + 26312976*x^6 + ...
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MAPLE
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a:= proc(d::nonnegint) option remember; if d = 1 then 1 else
add(a(k)*a(d-k)*(k^2*(d-k)^2*binomial(3*d-4, 3*k-2)-k^3*(d-k)
*binomial(3*d-4, 3*k-1)), k = 1 .. d-1) fi
end:
seq(a(n), n=1..20);
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MATHEMATICA
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a[n_] := a[n] = Sum[ a[k]*a[n-k]*k^2*(n-k)*(3k-n)*(3n-4)! / (3k-1)! / (3*(n-k)-2)!, {k, 1, n-1}]; a[1] = 1; Table[a[n], {n, 1, 13}] (* Jean-François Alcover, Nov 09 2011, after PARI *)
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PROG
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(PARI) {a(n) = if( n<2, n>0, sum(k=1, n-1, a(k) * a(n-k) * k^2 * (n-k) * (3*k-n) * (3*n-4)! / ((3*k-1)! * (3*(n-k)-2)!) ))}; /* Michael Somos, Dec 11 1999 */
(PARI)
N=20;
MEM=vector(N, j, -1); \\ for memoization
MEM[1] = 1;
K(d)= \\ Kontsevich's recursion, see S. Finch link.
{
my(m = MEM[d]);
if ( m != -1, return(m) ); \\ memoized
my(t, d2);
t = sum(d1=1, d-1, d2=d-d1; \\ d1+d2==d, both >= 1
K(d1) * K(d2) *
(d1^2 * d2^2 * binomial(3*d-4, 3*d1-2) -
d1^3 * d2^1 * binomial(3*d-4, 3*d1-1) )
);
MEM[d] = t; \\ memoize
return(t);
}
vector(N, d, K(d) )
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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Gary Kennedy (kennedy(AT)math.ohio-state.edu)
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EXTENSIONS
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STATUS
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approved
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