Lucas atoms

Given two variables $s$ and $t$, the associated sequence of Lucas polynomials is defined inductively by ${0}=0$, ${1}=1$, and ${n}=s{n-1}+t{n-2}$ for $nge2$. An integer (e.g., a Catalan number) defined by an expression of the form $prod_i n_i/prod_j k_j$ has a Lucas analogue obtained by replacing each factor with the corresponding Lucas polynomial. There has been interest in deciding when such expressions, which are a priori only rational functions, are actually polynomials in $s,t$. The approaches so far have been combinatorial. We introduce a powerful algebraic method for answering this question by factoring ${n}=prod_{d|n} P_d(s,t)$, where we call the polynomials $P_d(s,t)$ Lucas atoms. This permits us to show that the Lucas analogues of the Fuss-Catalan and Fuss-Narayana numbers for all irreducible Coxeter groups are polynomials in $s,t$. Using gamma expansions, a technique which has recently become popular in combinatorics and geometry, one can show that the Lucas atoms have a close relationship with cyclotomic polynomials $Phi_d(q)$. Certain results about the $Phi_d(q)$ can then be lifted to Lucas atoms. In particular, one can prove analogues of theorems of Gauss and Lucas, deduce reduction formulas, and evaluate the $P_d(s,t)$ at various specific values of the variables.