  We obtain explicit expressions for the class in the Grothendieck group of
varieties of the moduli space of genus 0 stable curves with n marked points.
This information is equivalent to the Poincar\'e polynomial; it implies
explicit expressions for the Betti numbers of the moduli space in terms of
Stirling numbers or, alternatively, Bernoulli numbers.
  The expressions are obtained by solving a differential equation
characterizing the generating function for the Grothendieck class as shown in
work of Yuri Manin from the 1990s. This differential equation is equivalent to
S. Keel's recursion for the Betti numbers of these moduli spaces. Our proof
reduces the solution to two combinatorial identities which follow from
applications of Lagrange series.
  We also study generating functions for the individual Betti numbers. In
previous work it had been shown that these functions are determined by a set of
polynomials with positive rational coefficients, which are conjecturally
log-concave. We verify this conjecture for many infinite families of these
polynomials, corresponding to the generating functions for the $2k$-Betti
numbers of the moduli spaces for all $k\le 100$. Further, studying these
polynomials allows us to prove that the generating function for the
Grothendieck class of the moduli spaces may be written as a series of rational
functions in the Lefschetz motive and the principal branch of the Lambert
W-function.
  We include an interpretation of the main result in terms of Stirling matrices
and a discussion of the Euler characteristic of the moduli space.
