Published online by Cambridge University Press: 18 November 2013
Finding the so-called characteristic numbers of the complex projective plane  $ \mathbb{C} {P}^{2} $ is a classical problem of enumerative geometry posed by Zeuthen more than a century ago. For a given
$ \mathbb{C} {P}^{2} $ is a classical problem of enumerative geometry posed by Zeuthen more than a century ago. For a given  $d$ and
$d$ and  $g$ one has to find the number of degree
$g$ one has to find the number of degree  $d$ genus
$d$ genus  $g$ curves that pass through a certain generic configuration of points and at the same time are tangent to a certain generic configuration of lines. The total number of points and lines in these two configurations is
$g$ curves that pass through a certain generic configuration of points and at the same time are tangent to a certain generic configuration of lines. The total number of points and lines in these two configurations is  $3d- 1+ g$ so that the answer is a finite integer number. In this paper we translate this classical problem to the corresponding enumerative problem of tropical geometry in the case when
$3d- 1+ g$ so that the answer is a finite integer number. In this paper we translate this classical problem to the corresponding enumerative problem of tropical geometry in the case when  $g= 0$. Namely, we show that the tropical problem is well posed and establish a special case of the correspondence theorem that ensures that the corresponding tropical and classical numbers coincide. Then we use the floor diagram calculus to reduce the problem to pure combinatorics. As a consequence, we express genus 0 characteristic numbers of
$g= 0$. Namely, we show that the tropical problem is well posed and establish a special case of the correspondence theorem that ensures that the corresponding tropical and classical numbers coincide. Then we use the floor diagram calculus to reduce the problem to pure combinatorics. As a consequence, we express genus 0 characteristic numbers of  $ \mathbb{C} {P}^{2} $ in terms of open Hurwitz numbers.
$ \mathbb{C} {P}^{2} $ in terms of open Hurwitz numbers.
 ${ \mathbb{R} }^{2} $, J. Amer. Math. Soc.  18  (2005), 313–377.CrossRefGoogle Scholar
${ \mathbb{R} }^{2} $, J. Amer. Math. Soc.  18  (2005), 313–377.CrossRefGoogle Scholar $\mathbf{Q} $-divisors on Kontsevich’s moduli space
$\mathbf{Q} $-divisors on Kontsevich’s moduli space  ${ \overline{M} }_{0, n} ({\mathbf{P} }^{r} , d)$ and enumerative geometry, Trans. Amer. Math. Soc.  351  (1999), 1481–1505.Google Scholar
${ \overline{M} }_{0, n} ({\mathbf{P} }^{r} , d)$ and enumerative geometry, Trans. Amer. Math. Soc.  351  (1999), 1481–1505.Google Scholar $\sum 1/ {n}^{2} $ by solving triangles, Amer. Math. Monthly  115  (2008), 745–752.CrossRefGoogle Scholar
$\sum 1/ {n}^{2} $ by solving triangles, Amer. Math. Monthly  115  (2008), 745–752.CrossRefGoogle Scholar