Type: Book-Chapter
Publication Date: 2010-01-01
Citations: 12
DOI: https://doi.org/10.1007/978-3-642-14444-8_15
The problem in the title seemed to be out of reach of any methods before 2004. We have still no answer for it, and it is no surprise that we will not answer it in the present work either. However, in the last few years the following developments arose in connection with the above problem. Supported by OTKA Grants K72731, K67676 and ERC-AdG.228005.