We say a symmetric divisor on M0,n is a symmetric F-divisor if D . FI1,I2,I3,I4 ≥0 for every F curve.
In the example below, we see that for n=8, the divisor 3B2+2B3+4B4 is a symmetric F-divisor, while the divisor B2 is not.
i1 : D=symmetricDivisorM0nbar(8,3*B_2+2*B_3+4*B_4)
o1 = SymmetricDivisorM0nbar{2 => 3 }
3 => 2
4 => 4
NumberOfPoints => 8
o1 : SymmetricDivisorM0nbar
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i2 : isSymmetricFDivisor(D) o2 = true |
i3 : D=symmetricDivisorM0nbar(8,B_2)
o3 = SymmetricDivisorM0nbar{2 => 1 }
NumberOfPoints => 8
o3 : SymmetricDivisorM0nbar
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i4 : isSymmetricFDivisor(D)
This divisor has negative intersection with the F curve F_{3, 2, 2, 1} (and maybe others too)
o4 = false
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