arXiv1995
Let $V$ be a finite dimensional vector space. Given a decomposition $V\otimes V=\oplus_i^n I_i$, define $n$ quadratic algebras $(V, J_m)$ where $J_m=\oplus_{i\neq m} I_i$. This decomposition defines also the quantum semigroup $M(V;I_1,...,I_n)$ which acts on all these quadratic algebras. With the decomposition we associate a family of associative algebras $A_k=A_k(I_1,...I_n)$, $k\geq 2$. In the classical case, when $V\otimes V$ decomposes into the symmetric and skewsymmetric tensors, $A_k$ coincides with the group algebra of the symmetric group $S_k$. Let $I_{ih}$ be deformations of the subspaces $I_i$. In the paper we give a criteria for flatness of the corresponding deformations of the quadratic algebras $(V[[h]],J_{ih}$ and the quantum semigroup $M(V[[h]];I_{1h},...,I_{nh})$. It says that the deformations will be flat if the algebras $A_k(I_1,...,I_n)$ are semisimple and under the deformation their dimension does not change. Usually, the decomposition into $I_i$ is defined by a given Yang-Baxter operator $S$ on $V\otimes V$, for which $I_i$ are its eigensubspaces, and the deformations $I_{ih}$ are defined by a deformation $S_h$ of $S$. We consider the cases when $S_h$ is a deformation of Hecke or Birman-Wenzl symmetry, and also the case when $S_h$ is the Yang-Baxter operator which appears by a representation of the Drinfeld-Jimbo quantum group. Applying the flatness criteria we prove that in all these cases we obtain flat deformations of the quadratic algebras and the corresponding quantum semigroups.