Abstract. As a first goal, it is explained why the Goodwillie-Weiss calculus of embeddings offers new information about the Euclidean embedding dimension of P m only for m ≤ 15. Concrete scenarios are described in these low-dimensional cases, pinpointing where to look for potential-but criticalhigh-order obstructions in the corresponding Taylor towers. For m ≥ 16, the relation TC S (P m ) ≥ n is translated into the triviality of a certain cohomotopy Euler class which, in turn, becomes the only Taylor obstruction to producing an embedding P m ⊂ R n . A speculative bordism-type form of this primary obstruction is proposed as an analogue of Davis' BP -approach to the immersion problem of P m . A form of the Euler class viewpoint is applied to show that TC S (P 3 ) = 5, as well as to suggest a few higher-dimensional projective spaces for which the method could produce new information. As a second goal, the paper extends Farber's work on the motion planning problem in order to develop the notion of a symmetric motion planner for a mechanical system S. Following Farber's lead, this concept is connected to TC S (C(S)), the symmetric topological complexity of the state space of S. The paper ends by sketching the construction of a concrete 5-local-rules symmetric motion planner for P 3 .
Main results
Recall the Schwarz genus genus(p) of a fibration p : E → B ([26]); it is one less 1 than the smallest number of open sets U covering B in such a way that p admits a (continuous) section over each U .
Definition 1.1 ([8]). The topological complexity of a space X, TC(X), is defined as the genus of the end-points evaluation map ev : P (X) → X × X, where P (X) is the free path space X [0,1] with the compact-open topology.Let F (X, 2) ⊂ X × X denote the configuration space of ordered pairs of distinct points in X, and ev 1 : P 1 (X) → F (X, 2) be the restriction of the fibration ev. Thus P 1 (X) is the subspace of P (X) obtained by removing the free loops on X. The group Z/2 acts freely on both P 1 (X) and F (X, 2), by running a path backwards in