各位大侠，谁能告诉我风险度量的封闭的近似值   或者 封闭解 怎么理解？？  万分感激！！   

Analytic bounds and approximations for annuities and Asian options
Steven Vanduffela,, Zhaoning Shanga, Luc Henrardb, Jan Dhaenea, Emiliano A. Valdezc
a Katholieke Universiteit Leuven, Belgium
b Fortis, Belgium
c University of New South Wales, Australia
Received August 2007; received in revised form February 2008; accepted 20 February 2008

我是看这篇文献的时候出来的问题