Published online by Cambridge University Press: 20 November 2018
Following Radford's proof of Lagrange's theorem for pointed Hopf algebras, we prove Lagrange‘s theorem for Hopf monoids in the category of connected species. As a corollary, we obtain necessary conditions for a given subspecies   $\mathbf{k}$  of a Hopf monoid
 $\mathbf{k}$  of a Hopf monoid   $\mathbf{h}$  to be a Hopf submonoid: the quotient of any one of the generating series of
 $\mathbf{h}$  to be a Hopf submonoid: the quotient of any one of the generating series of   $\mathbf{h}$  by the corresponding generating series of
 $\mathbf{h}$  by the corresponding generating series of   $\mathbf{k}$  must have nonnegative coefficients. Other corollaries include a necessary condition for a sequence of nonnegative integers to be the dimension sequence of a Hopf monoid in the form of certain polynomial inequalities and of a set-theoretic Hopf monoid in the form of certain linear inequalities. The latter express that the binomial transform of the sequence must be nonnegative.
 $\mathbf{k}$  must have nonnegative coefficients. Other corollaries include a necessary condition for a sequence of nonnegative integers to be the dimension sequence of a Hopf monoid in the form of certain polynomial inequalities and of a set-theoretic Hopf monoid in the form of certain linear inequalities. The latter express that the binomial transform of the sequence must be nonnegative.