The isochronous center problem for polynomial systems is an open problem .
时间可逆多项式系统的等时中心问题是一类公开问题.
The dissolution of hu-han contexts is not an isochronous progress together with the flourishing and perdition of a dynasty ; on the one hand it is a natural result of national conflicts and amalgamations ; on the other hand , the evolvement of human 's notions is a vital factor of the dissolution .
