Genus outputs -1 sometimes

[sagetrac-bthompson]

Sage outputs -1 as the genus of a curve, it should probably throw an error instead. The genus function doesn't seem to check if the function is irreducible, and it just attempts to calculate the genus anyway.

sage: FF.<c>  =QQ[] 
sage: R.<x> = FF[]
sage: K = QQ[c, x]
sage: poly2 = K(x^2 - c^4)
sage: C2 = Curve(poly2)
sage: C2.genus()
-1

CC: @bhutz @slel

Component: elliptic curves

Keywords: sagedays103, dynamics, curves

Issue created by migration from https://trac.sagemath.org/ticket/28336

[sagetrac-bthompson]

Changed keywords from sagedays103, dynamics to sagedays103, dynamics, curves

[embray]

comment:3

Ticket retargeted after milestone closed

[mkoeppe]

comment:4

Moving tickets to milestone sage-9.2 based on a review of last modification date, branch status, and severity.

[mkoeppe]

comment:7

Moving to 9.4, as 9.3 has been released.