If status is good on entry this routine will calculate the 2nd order differential of chi-squared with respect to A(J). It does this numerically by calling the supplied routine XISQ_ROUTINE to calculate chi-squared at the current parameter values, then again at A(J)$+$DELTA_AJ and A(J)-DELTA_AJ, then using the formula:-

$$\frac{{\partial }^2{\chi }^2}{\partial A{J}^2}=\frac{{\chi }^2\left(AJ+\delta AJ\right)-2{\chi }^2\left(AJ\right)+{\chi }^2\left(AJ-\delta AJ\right)}{\delta A{J}^2}$$

(41)

DELTA_AJ is equal to the absolute value of 0.001 $\ast$ A(J) or, if this is zero, 0.001.

CALL SCULIB_FIT_D2XISQ_DAJ2 (XISQ_ROUTINE, N, A, J, D2XISQ_DAJ2, STATUS)

XISQ_ROUTINE (XISQ, N, A, STATUS) = EXTERNAL ROUTINE (Given)

N = INTEGER (Given)

A(N) = DOUBLE PRECISION (Given and returned)

J = INTEGER (Given)

D2XISQ_DAJ2 = DOUBLE PRECISION (Returned)

STATUS = INTEGER (Given and returned)

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