Realization conjecture for the planar binary tree Hopf algebra

Let Sn\mathfrak S_n be the symmetric group, let (eσ)σSn(e_\sigma)_{\sigma\in\mathfrak S_n} be a basis of AnA_n, and let PT\mathbf P_T denote the polynomial associated with a planar binary tree TT of size nn. Let ρm,n\rho_{m,n} be the morphism associated with the tower, let K=n0K0(An)\mathcal K=\bigoplus_{n\geq 0}K_0(A_n), and define the induction product by

[M][N]=[MCNAmAnAm+n].[M]\cdot[N]=[M\otimes_{\mathbb C}N\mathord\uparrow_{A_m\otimes A_n}^{A_{m+n}}].

Realization conjecture. There exists a tower of algebras AnA_n, with a basis (eσ)σSn(e_\sigma)_{\sigma\in\mathfrak S_n}, such that the restriction of ρm,n\rho_{m,n} to canonical words is given by the product of the corresponding PT\mathbf P_T functions; the indecomposable projective modules of AnA_n are the left ideals PT=AneσTP_T=A_ne_{\sigma_T} and are therefore in bijection with planar binary trees of size nn; and the map

KPBT\mathcal K\longrightarrow\mathbf{PBT}

sending the class of PTP_T to PT\mathbf P_T is a ring isomorphism. This conjecture would provide an algebraic realization of the planar binary tree Hopf algebra, refining the existence conjecture and identifying its projective representation theory with the basis of planar binary trees. The source provides no evidence of resolution.

Sources & referencesView supporting material

Primary source

F. Hivert, J. -C. Novelli and J. -Y. Thibon, “The Algebra of Binary Search Trees”, arXiv:math/0401089 (2004).

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