Canonical characters on quasi-symmetric functions and bivariate Catalan   numbers
  Every character on a graded connected Hopf algebra decomposes uniquely as a product of an even character and an odd character (Aguiar, Bergeron, and Sottile, math.CO/0310016).   We obtain explicit formulas for the even and odd parts of the universal character on the Hopf algebra of quasi-symmetric functions. They can be described in terms of Legendre's beta function evaluated at half-integers, or in terms of bivariate Catalan numbers:   $$ C(m,n)=\frac{(2m)!(2n)!}{m!(m+n)!n!}. $$   Properties of characters and of quasi-symmetric functions are then used to derive several interesting identities among bivariate Catalan numbers and in particular among Catalan numbers and central binomial coefficients. 
Marcelo Aguiar and Samuel K. Hsiao