Continuous self-similarity in parametric piecewise isometries

Abstract: We exhibit two distinct renormalization scenarios in many-parameter families of piecewise isometries (PWI) of a rhombus. The rotational component, defined over the quadratic field $\mathbb{K}=\mathbb{Q}(\sqrt{5})$, is fixed. The translations are specified by affine functions of the parameters, with coefficients in $\mathbb{K}$. In each case the parameters range over a convex domain. In one scenario the PWI is self-similar if and only if one parameter belongs to $\mathbb{K}$, while the other is free. Such a continuous self-similarity is due to the possibility of merging adjacent atoms of an induced PWI, a common phenomenon in the Rauzy-Veech induction for interval exchange transformations. In the second scenario, the phase space splits into several disjoint (non-convex) invariant components. We show that each component has continuous self-similarity, but due to the transversality of the corresponding foliations, full self-similarity in phase space is achieved if and only if both parameters belong to $\mathbb{K}$. All our computations are exact, using algebraic numbers.