Traces for Besov spaces on fractal h-sets and dichotomy results

We study the existence of traces of Besov spaces on fractal $h$-sets $\Gamma$ with the special focus laid on necessary assumptions implying this existence, or, in other words, present criteria for the non-existence of traces. In that sense our paper can be regarded as an extension of [Br4] and a continuation of the recent paper [Ca2]. Closely connected with the problem of existence of traces is the notion of dichotomy in function spaces: We can prove that -- depending on the function space and the set $\Gamma$ -- there occurs an alternative: either the trace on $\Gamma$ exists, or smooth functions compactly supported outside $\Gamma$ are dense in the space. This notion was introduced by Triebel in [Tr7] for the special case of $d$-sets.