All Forums  Origin Forum  Origin Forum  Problem with my own exp fitting function

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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 marasneo[/i] [br]Hi! I have tried to modify standard Origin ExpDecay2 function to solve equation like this: Z=C0+C1*exp(x1*t)+C2*exp(x2*t) where t is my x (independent variable) and Z is dependent (y) I am interested in determination of x1 and x2 roots which should be always negative. Unfortunately, after fitting, they are always positive. Probably I am doing something wrong... There is my modified code below (probably with errors). I need to take into account also the offset x0 (I do not know if the formula will be the same as for ExpDecay2 or not). Always before each fitting I am just setting the Auto Parameter Initialization and checking "fixed" for C0 and x0 offset values (the same for function like ExpDecay2 - is it ok?). FUNCTION C0 + C1*exp(-(x-x0)*ex1) + C2*exp(-(x-x0)*ex2) PARAMETER INIT x0 = min(x_data); int sign; ex1 = get_exponent(x_data, y_data, &C0, &C1, &sign); if (ex1 > 0) { int nSize = x_data.GetSize(); C0 = y_data[0] + y_data[nSize-1] - C0; C1 = C2 = -0.5 * sign * exp(C1+ex1*(x_data[0]+x_data[nSize-1])-x0*ex1); ex1 = ex2 = 1 / ex1; } else { ex1 = ex2 = -1 / ex1; C1 = C2 = 0.5 * sign * exp(C1 - x0/ex1); } Please, help me with this problem. [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.