Zeuthen–Segre invariant
In algebraic geometry, the Zeuthen–Segre invariant "I" is an invariant of a projective surface found in a complex projective space which was introduced by and rediscovered by . The invariant "I" is defined to be "d" – 4"g" – "b" if the surface has a pencil of curves, non-singular of genus "g" except for "d" curves with 1 ordinary node, and with "b" base points where the curves are non-singular and transverse.