Counting planar curves in P3 with degenerate singularities

Abstract In this paper, we consider the following question: how many degree d curves are there in P 3 (passing through the right number of generic lines and points), whose image lies inside a P 2 , having δ nodes and one singularity of codimension k? We obtain an explicit formula for this number when δ + k ≤ 4 (i.e. the total codimension of the singularities is not more than four). We use a topological method to compute the degenerate contribution to the Euler class; it is an extension of the method that originates in the paper by A. Zinger [30] and which is further pursued by S. Basu and the second author in [1] , [2] and [3] . Using this method, we have obtained formulas when the singularities present are more degenerate than nodes (such as cusps, tacnodes and triple points). When the singularities are only nodes, we have verified that our answers are consistent with those obtained by S. Kleiman and R. Piene (in [17] ) and by T. Laarakker (in [19] ). We also verify that our answer for the characteristic number of planar cubics with a cusp and the number of planar quartics with two nodes and one cusp is consistent with the answer obtained by R. Singh and the second author (in [22] ), where they compute the characteristic number of rational planar curves in P 3 with a cusp. We also verify some of the numbers predicted by the conjecture made by Pandharipande in [23] , regarding the enumerativity of BPS numbers for P 3 .