Note that the subscript letters 'm', 'i', 'j', 'k' and 'n' in the abstract have not copied correctly and the *'s denote symbols that use letters in both subscript and superscript that do not copy correctly.
RGMIA digitization. CAVAL 2011
Abstract amended to exclude formula.  Lyn 10/2/12
Permission granted, in perpetuity, RGMIA version. liz 6/9/12