I got this error msg for this line of code: n = -(2(a**3.0)/27.0 - a*b/3.0 + c) (where a = 1, b = 2, c = 3) And was baffled until I realized the line should be n = -(2*(a**3.0)/27.0 - a*b/3.0 + c) But I still don't understand what "callable" means. Can someone help? Thanks, Dick Moores rdm at rcblue.com