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c subroutine fderiv.f (non-analytical)
c
c source
c Bevington, page 242.
c
c purpose
c evaluate derivatives of function for least-squares search
c for arbitrary function given by functn
c
c usage
c call fderiv (x, i, a, deltaa, nterms, deriv)
c
c description of parameters
c x - array of data points for independent variable
c i - index of data points
c a - array of parameters
c deltaa - array of parameter increments
c nterms - number of parameters
c deriv - derivatives of function
c
c subroutines and function subprograms required
c functn (x, i, a)
c evaluates the fitting function for the ith term
c
subroutine fderiv (x,i,a,deltaa,nterms,deriv)
dimension x(1),a(1),deltaa(1),deriv(1)
real FUNCTN
external FUNCTN
11 do 18 j=1,nterms
aj=a(j)
delta=deltaa(j)
a(j)=aj+delta
yfit=functn(x,i,a)
a(j)=aj-delta
deriv(j)=(yfit-functn(x,i,a))/(2.*delta)
18 a(j)=aj
return
end
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