Single Basepoint Subdivision Schemes for Manifold-valued Data: Time-Symmetry Without Space-Symmetry

This paper establishes smoothness results for a class of nonlinear subdivision schemes, known as the single basepoint manifold-valued subdivision schemes, which shows up in the construction of wavelet-like transform for manifold-valued data. This class includes the (single basepoint) Log–Exp subdivision scheme as a special case. In these schemes, the exponential map is replaced by a so-called retraction map f from the tangent bundle of a manifold to the manifold. It is known that any choice of retraction map yields a C2 scheme, provided the underlying linear scheme is C2 (this is called “C2 equivalence”). But when the underlying linear scheme is C3, Navayazdani and Yu have shown that to guarantee C3 equivalence, a certain tensor Pf associated to f must vanish. They also show that Pf vanishes when the underlying manifold is a symmetric space and f is the exponential map. Their analysis is based on certain “Ck proximity conditions” which are known to be sufficient for Ck equivalence.In the present paper, a geometric interpretation of the tensor Pf is given. Associated to the retraction map f is a torsion-free affine connection, which in turn defines an exponential map. The condition Pf=0 is shown to be equivalent to the condition that f agrees with the exponential map of the connection up to the third order. In particular, when f is the exponential map of a connection, one recovers the original connection and Pf vanishes. It then follows that the condition Pf=0 is satisfied by a wider class of manifolds than was previously known. Under the additional assumption that the subdivision rule satisfies a time-symmetry, it is shown that the vanishing of Pf implies that the C4 proximity conditions hold, thus guaranteeing C4 equivalence. Finally, the analysis in the paper shows that for k≥5, the Ck proximity conditions imply vanishing curvature. This suggests that vanishing curvature of the connection associated to f is likely to be a necessary condition for Ck equivalence for k≥5.