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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by vanhufau[/i] [br]I am trying to fit data with a Hill´s function. I have defined this function, and I can´t find any problem in the expression. I have 12 parameters, but each time I try to fit, it says :"try to set parameter ... as fixed", and it works only when more than 7 parameters are fixed. Why????? My function is : y=2*mu1*(x^a1-(x*(1.35275-0.36352*x)^2)^(-a1*b1))/(a1*x) + 2*mu2*(x^a2-(x*(1.35275-0.36352*x)^2)^(-a2*b2))/(a2*x) + 2*mu3*(x^a3-(x*(1.35275-0.36352*x)^2)^(-a3*b3))/(a3*x) + 2*mu4*(x^a4-(x*(1.35275-0.36352*x)^2)^(-a4*b4))/(a4*x) [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.