Does anyone have a reference for the invariant binary forms of a quintic? That is, what are the $SL_2(C)$ invariant polynomial functions on the space of binary quintics.
According to this page there are independent invariants of degree 4, 8, and 12, plus a degree-18 invariant whose square is a polynomial in the first three. This is attributed to Gordan (1868):
That page also describes the invariants, and even the covariants, for several other degrees, and also variations such as multiple forms.
[All I did was Google it; the present MO question already turned up in the first page of search results only a few hours after it was posted.]