the ox, oy, oz things seems to be booleans which are here to "activate" a derivative operator. ox=%T, oy=%F, oz = %T then the expression is: 2 d dfp(i) = ----- s (xp(i),yp(i),zp(i)) dx.dz I will try to find a good way to present this while I am converting text equations into mml. YC