To make my question precise, suppose you have a complex curve locally given by $$f(x,y) =0 $$

and $f$ has singularity of type $\chi_k$ at the origin. The codimension of this singularity is $k$. Let $g$ be the contribution of the singularity to the genus of that curve. For instance if it was a simple node ($A_1$) then $g=1$. If it was a tacnode ($A_3$) then $g$ is $2$ and so on. Is it true that

$$ g \leq k-2 $$

provided, $k \geq 5$ ? This does seem to be true if $k =5, 6,7$ because in these dimensions we know that the only type of singularities are A, D and E. We just need to verify for each of them. But beyond seven dimensions there is no classification of singularities. I do not wan to make any additional assumption on the singularity, such as being simple or anything else. Basically my question is can we give an upper bound for the genus (hopefully the one I am suggesting), without even having a classification? Similarly is there a lower bound for the genus? Intuitively, the meaning of $g$ is how many nodes you get after you change the singularity a bit. So if you deform a tacnode a bit, you get two nodes.


1 Answer 1


I assume you are talking about equisingular/topologically equivalent singularities (if you are talking about the analytical types, then the codimension is even higher). In that case, the relation can be computed from the embedded resolution of the singularity, as follows. This resolution consists in blowing up the point, and as long as the obtained curve plus the exceptional divisor does not have Normal Crossings, keep blowing up the points at which this fails. For each point $p$ that has to be blown up, let $m_p$ be the multiplicity of the curve at $p$; and let $f$ be the total number of non-satellite points to be blown up (a point is satellite if it is the intersection point of two exceptional components of previous blowups). Then $k=g+\sum m_p-f-1$ [Edit: because both $k$ and $g$ can be computed from the resolution; see Kleiman-Piene, Enumerating singular curves on surfaces, in "Algebraic geometry: Hirzebruch 70" and references therein. In Kleiman-Piene, $k$ is called "cod" and $g$ is called $\delta$.]

Edit: as soon as the singularity has multiplicity $\ge 4$, $\sum m_p-f-1\ge 4-1-1$ and your lower bound follows. If it has multiplicity 3 and at least one more point in the resolution has multiplicity 3, then $\sum m_p-f-1\ge 6-2-1$. All remaining types are A, D or E and hence covered by your argument so the upper bound as you claimed.

If you want a lower bound of the type $g\ge k- constant$ (I had not noticed this part of the question, sorry) then the formula tells you it is impossible. For instance, the ordinary singular point of multiplicity m has $k=g+m-2$ and $m$ can be arbitrarily large. On the other hand, since both $k$ and $g$ grow quadratically with $m$, you can surely get bounds of the form $g \ge k \times constant$ (and pick the constant arbitrarily close to 1 by restricting to large $k$). (If you are interested in the codimension of the equianalytic stratum, this argument does not work, but I still believe a lower bound of this sort may exist).

  • $\begingroup$ Thank you for your answer. Is there any reference you can point out to me where I will get this result? And I assume you are saying both my bounds follow? The upper bound and the lower bound? $\endgroup$
    – Ritwik
    Oct 13, 2011 at 22:43
  • $\begingroup$ I am sorry, the lower bound can not possibly follow from that equation! $\endgroup$
    – Ritwik
    Oct 14, 2011 at 0:36
  • $\begingroup$ I modified the answer to deal with your comments, hope it is clearer now. $\endgroup$
    – quim
    Oct 14, 2011 at 8:57
  • $\begingroup$ Thank you for the reference. I need to be sure of something. Is codimension, intuitively the ``number of conditions'' needed to specify the singularity? For instance I would say a tacnode (A_3) is codimension 3 because you need 3 conditions to specify a tacnode. From their defenition of codimension on Page 214, it doesn't seem to be immediately obvious. $\endgroup$
    – Ritwik
    Oct 14, 2011 at 22:27
  • $\begingroup$ Yes. Their formula counts the number of conditions imposed by the singularity in a fixed position of the points of the resolution, and then subtracts the degrees of freedom for the positions of these points, resulting in the codimension you refer to. Maybe you'd find interesting their more recent work, arXiv:0905.2169, appeared I think in Rendiconti Lincei. $\endgroup$
    – quim
    Oct 15, 2011 at 6:56

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