求助两篇英文文献：

1. Bivariate symmetry: definitions and basic properties
Ibrahim A. Ahmad; Patricia B. Cerrito
Journal of Nonparametric Statistics, Volume 1, Issue 1, 1991, Pages 165 – 169

2. some concepts of bivariate symmetry .

  Nelsen. RB.  Journal of Nonparametric Statistics, Volume 3, 1993, Pages 95 – 101.

谢谢！