Philida (André P. Brink)

Se p 20 03 What is spin ? André Gsponer

This is a late answer to question #79 by R.I. Khrapko, “Does plane wave not carry a spin?,” Am. J. Phys. 69, 405 (2001), and a complement (on gauge invariance, massive spin 1 and 1 2 , and massless spin 2 fields) to the paper by H.C. Ohanian, What is spin?, Am. J. Phys. 54, 500–505 (1985). In particular, it is confirmed that “spin” is a classical quantity which can be calculated for any field u...

Back from the brink

Against the backdrop of a global loss of biodiversity, some wildlife species have recovered from severe decimation, as a recent report shows for dozens of species in Europe. While these encouraging signs don't diminish the current crisis of species loss, the analysis of the underlying reasons--ranging from targeted conservation measures to unintended consequences of changes in human behaviour--...

A conjecture of Yves André . ∗

The introduction to [2] (and references contained therein) contains a comprehensive exposition of terminology and notations relative to this conjecture. Since we use the same terminology and notations, we do not reproduce them here. The introduction to [2] also contains an exposition of results on this conjecture obtained before the article [3] came out. In this article we prove the following t...

François Bach and André Herbelin

Reals which Compute Little André

We investigate combinatorial lowness properties of sets of natural numbers (reals). The real A is super-low if A′ ≤tt ∅′, and A is jump-traceable if the values of {e}A(e) can be effectively approximated in a sense to be specified. We investigate those properties, in particular showing that super-lowness and jump-traceability coincide within the r.e. sets but none of the properties implies the o...