中国科学院数学与系统科学研究院期刊网
AMS
On Self-Adjointness of the Product of Two Second-Order Differential Operators
Zhijiang Cao, Jiong Sun, D. E. Edmunds
Acta Mathematica Sinica . 1999, (3): 375 -386 .  DOI: 10.1007/BF02650732