It looks like where the argument of is . However, I don't know how to parse the nominator, namely, . I am sure your program has issues with this as well.y=a+b(1-exp(-((x+dln(2)1/e-c)/d)e))

One thing that may be necessary is to insert the multiplication signs. E.g., write a + b * (1 - exp(...)) instead of a + b(1 - exp(...)).