In a message dated 5/5/00 10:30:52 PM Central Daylight Time, tesla-at-pupman-dot-com writes: << k=(b^2-a^2)/(b^2+a^2), where a and b are integers with (b-a) odd >> Antonio, Would you please explain how this equation is derived. Do a and b have dimensions? Can you give refereces for the texts you refer to? Cheers, Ralph Zekelman