15.37.3 EQUATION

LINEAR This allows you to enter equations for each term of a series to fit. Each term is separated by a semicolon ";". The zeroeth order term is always assumed to be a constant so only the first, second... order terms are entered. For example you wish to fit Y=C0+C1*X**1.5+C2*EXP(X) TD:FIT EQUATION "XV**1.5;EXP(XV)" If you have previously fit using an equation: TD:FIT EQUATION "" Uses the same equation for a new fit. All other fit options may be used with EQUATION, but any options that modify the number of terms to fit must appear after the equation definition. The X offset is ignored by the equation. NONLINEAR If you wish to perform a fit to an arbitrary function of X, then you must enter the coefficients in the the equation as COE[n], and you must specify the NONLINEAR option. Remember that each term automatically has a coefficient, so you should use coefficients in the equation that have not been already used. For example you wish to fit the equation: C0+C1*/(1+C2*(X-C3)**2) Recalling that C0, and C1 will already be used you enter: TD:FIT EQUATION "1/(1+COE[2]*(XV-COE[3])**2)" NONLINEAR TERMS=3 You may also need to specify initial values for the coefficients and inital errors for the first step. CONSTRAIN=OFF COEF n0,n1,n2,n3 ERROR n0,n1,n2,n3 Note: The constant term COE[0] is fit by default. Likewise the C1 is automatically included as the coefficient of the first term. To get rid of the constant term you use the option: EXCLUDE 0