Note that the * denotes a complicated mathematical fraction using letters in both superscript and subscript, the fractions have been written on a single line and the superscript letters 'x' and 'N' have not copied correctly.
RGMIA digitization. CAVAL 2011.  Abstract removed  Lyn 26/9/12. Permission granted, in perpetuity, RGMIA verson. liz 6/9/12