Vietnam Journal of Mathematics
Vietnam Journal of Mathematics > 2018 > 46 > 2 > 259-269
Vietnam Journal of Mathematics > 2018 > 46 > 4 > 1023-1051
Vietnam Journal of Mathematics > 2018 > 46 > 2 > 259-269
Vietnam Journal of Mathematics > 2018 > 46 > 4 > 1023-1051