I have the following code: #!/usr/bin/env python from scipy.optimize import fsolve import math h = 6.634e-27 k = 1.38e-16 freq1 = 88633.9360e6 freq2 = 88631.8473e6 freq3 = 88630.4157e6 def J(freq,T): return (h*freq/k)/(math.exp(h*freq/(k*T))-1) def equations(x,y,z,w,a,b,c,d): f1 = a*(J(freq1,y)-J(freq1,2.73))*(1-math.exp(-a*z))-(J(freq2,x)-J(freq2,2.73))*(1-math.exp(-z)) f2 = b*(J(freq3,w)-J(freq3,2.73))*(1-math.exp(-b*z))-(J(freq2,x)-J(freq2,2.73))*(1-math.exp(-z)) f3 = c*(J(freq3,w)-J(freq3,2.73))*(1-math.exp(-b*z))-(J(freq1,y)-J(freq1,2.73))*(1-math.exp(-a*z)) f4 = d*(J((freq3+freq1)/2,(y+w)/2)-J((freq3+freq1)/2,2.73))-(J(freq2,x)-J(freq2,2.73))*(1-math.exp(-z)) return (f1,f2,f3,f4) So, I have defined the equations in the above code. However, I now wish to solve the above set of equations using fsolve or other alternative non-linear numerical routine. I tried the following syntax but with no avail: x,y,z,w = fsolve(equations, (1,1,1,1)) I keep getting the error that "x" is not defined. I am executing all commands at the command-line, since I have no idea how to run a batch of commands as above automatically in python. I welcome any advice on how to solve this.