# Mathematics Colloquia and Seminars

With Li I have introduced a set of axioms which guarantee that the Grothendieck groups of a tower of algebras $\bigoplus_{n\ge0}A_n$ can be endowed with the structure of graded dual Hopf algebras. Hivert and Nzeutzhap, and independently Lam and Shimozono constructed dual graded graphs from primitive elements in Hopf algebras. With Li and Lam, I apply the composition of these constructions to towers of algebras. We show that if a tower $\bigoplus_{n\ge0}A_n$ gives rise to graded dual Hopf algebras then we must have $\dim(A_n)={r^n}n!$ where $r = \dim(A_1)$. This shows that Combinatorial Hopf algebras obtained by this procedure fall into a very rigid framework and can potentially be classified.