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3. Castelnuovo, G.: Numero delle involutione razionali giacenti sopra una curva di dato genere. Rend. R. Acad. Lincei, Ser 4,5 (1889)
4. Diaz, S.: Exceptional Weierstrass points and the divisor on moduli space that they define. Thesis, Brown University 1982
5. Eisenbud, D., Harris, J.: Divisors on general curves and cuspidal rational curves. Invent. Math.74, 371?418 (1983)