Maxima can't seem to evaluate numerically float(exp(exp(2))) . (C30) float(exp(exp(2))); 2 %E (D30) 2.718281828459045 I used the following workaround: y(x) := EXP(FLOAT(EXP(x))) ROMBERG(EXP(- EXP(s)), s, 1, x) Ugly!