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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include <math/surfit.h>
+include "surfitdef.h"
+
+# ISLSOLVE -- Procedure to  solve for the x coefficients of image line
+# number lineno. The inner products of the x basis functions are assumed
+# to be in the SF_XORDER(sf) by SF_NXCOEFF(sf) array XMATRIX,
+# while the inner products of the basis functions and
+# the data ordinated for line number lineno are assumed to be in the
+# lineno-th row of the SF_NXCOEFF(sf) by SF_NLINES(sf) matrix XCOEFF.
+# The Cholesky factorization of XMATRIX is calculated and placed
+# in XMATRIX overwriting the original data. The x coefficients for
+# line number lineno are calculated and placed in the lineno-th row
+# of XCOEFF replacing the original data.
+
+procedure islsolve (sf, lineno, ier)
+
+pointer	sf 		# pointer to the surface descriptor structure
+int	lineno		# line being fitted in x
+int	ier		# ier = 0, everything OK
+			# ier = 1, matrix is singular
+			# ier = 2, no degree of freedom
+
+pointer	xcptr
+
+begin
+	# return if there are insuffucient points to solve the matrix
+	ier = OK
+	if ((SF_NXPTS(sf) - SF_NXCOEFF(sf)) < 0 ) {
+	    ier = NO_DEG_FREEDOM
+	    return
+	}
+
+	# calculate the Cholesky factorization of the x matrix and store
+	# separately for possible use by SFLREFIT
+
+	call sfchofac (XMATRIX(SF_XMATRIX(sf)), SF_XORDER(sf), SF_NXCOEFF(sf),
+	    XMATRIX(SF_XMATRIX(sf)), ier)
+
+	# solve for the x coefficients for line lineno assuming the
+	# data are in row lineno of xcoeff, the solution is placed
+	# on top of the data
+
+	xcptr = SF_XCOEFF(sf) + (lineno - 1) * SF_NXCOEFF(sf)
+	call sfchoslv (XMATRIX(SF_XMATRIX(sf)), SF_XORDER(sf), SF_NXCOEFF(sf),
+	    XCOEFF(xcptr), XCOEFF(xcptr))
+end