Institute of Mathematics of The Academy of Sciences of the Czech Republic (published by Springer) Posted on July 8, 2014 by SCG DR Copyright Holders: Institute of Mathematics of The Academy of Sciences of the Czech Republic (published by Springer) Self-archiving policy Conditions Accepted version: √ Published version: √ 24 months embargo Remarks: DR-NTU can archive author’s post-print (accept version). It is possible for DR-NTU to archive publisher’s PDF version with 24 months embargo. (verified by Guang Ying via email) Verification Date: 24.06.2014