arXiv2026
For many groups, parabolic subgroups arise as stabilizers of flags of sets or vector spaces; the quotients by these subgroups parametrize orbits of flags, and entropies appear as rates of exponential or superexponential growth of their cardinalities. The multiplicative "chain rules" relating these cardinalities induce, asymptotically, additive analogues for the entropies. Traditional formulas in information theory correspond to quotients of symmetric groups, a particular kind of reflection group: orbit cardinalities are multinomial coefficients, related to Shannon entropy. Quotients of general linear groups over a finite field can be treated similarly, with $q$-multinomials in place of multinomials and the Tsallis 2-entropy in place of Shannon's. Here we consider other finite reflection groups and classical groups over finite fields (split groups of Lie type). In both settings the groups are classified by Dynkin diagrams into infinite series $A_n$, $B_n$, $C_n$, $D_n$ and finitely many exceptional ones; the $A_n$ series consists of the symmetric groups (reflection case) and general linear groups (Lie case). Our treatment is uniform, based on classical invariant theory: every finite reflection group $W$ has well-defined degrees, and the product of their $q$-deformations is the Poincaré polynomial of $W$ (Solomon's formula). Quotients of Poincaré polynomials count at once the cosets of parabolic subgroups of $W$ (at $q=1$) and the points of the flag varieties $G/P$ of the corresponding algebraic groups over $\mathbb F_q$ (at a prime power $q$). The series $B_n$, $C_n$, and $D_n$, studied here from an information-theoretic perspective for the first time, are linked to new entropic functionals: a reflective entropy governing the reflection groups, and a Lie entropy governing the associated groups of Lie type; each follows a distinctive chain rule.