The notion of (a,b)-cores is closely related to rational (a,b) Dyck paths due to Anderson's bijection, and thus the number of (a,a+1)-cores is given by the Catalan number Ca. Recent research shows that (a,a+1) cores with distinct parts are enumerated by another important sequence- Fibonacci numbers Fa. In this paper, we consider the abacus description of (a,b)-cores to introduce the natural grading and generalize this result to (a,as+1)-cores. We also use the bijection with Dyck paths to count the number of (2k−1,2k+1)-cores with distinct parts. We give a second grading to Fibonacci numbers, induced by bigraded Catalan sequence Ca,b(q,t).

Combining results of T.K. Lam and J. Stembridge, the type C Stanley symmetric function FCw(x), indexed by an element w in the type C Coxeter group, has a nonnegative integer expansion in terms of Schur functions. We provide a crystal theoretic explanation of this fact and give an explicit combinatorial description of the coefficients in the Schur expansion in terms of highest weight crystal elements.