Answering a question of Per Lindstrom, we show that there is no \plus-capping" degree, i.e. that for any incomplete r.e. degree w, there is an incomplete r.e. degree a > w such that there is no r.e. degree v > w with a \ v = w. 1. The Theorem. In [2], Feferman de nes relative interpretability for extensions of Peano arithmetic. The induced structure D T of degrees of interpretability turns out...
