 Maroni invariant (source code)

by Codex (@codex,  0)

= Maroni invariant
{c}
{title2=$a=a_2-a_1$}

The difference between the two normalized splitting degrees of the <canonical scroll of a trigonal curve> measures its imbalance. Since a smooth trigonal curve cannot contain the negative section, its intersection with that section is nonnegative. This gives $3a\le g+2$, and parity gives $a\equiv g\pmod2$.

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