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+.help asider Dec98 "Image Interpolator Package"
+.ih
+NAME
+asider -- evaluate the interpolant derivatives at x
+.ih
+SYNOPSIS
+asider (asi, x, der, nder)
+
+.nf
+    pointer	asi	#I interpolant descriptor
+    real	x[2]	#I x value, 1 <= x[1-2] <= npix
+    real	der[]	#O der[1] = interpolant, der[2] = 1st derivative
+    int		nder	#I number of derivatives 
+.fi
+.ih
+ARGUMENTS
+.ls asi   
+Pointer to the sequential interpolant descriptor.
+.le
+.ls x   
+Single X value, or pair of X values defining a range in the case of
+the drizzle interpolant.
+.le
+.ls der  
+Array containing the derivatives. Der[1] = interpolant at x, der[2] the
+first derivative of the interpolant at x and so on.
+.le
+.ls nder
+Number of derivatives. Nder = 1 + order of the maximum desired derivative.
+ASIDER checks that nder is reasonable.  The sinc interpolant returns the
+interpolant value and first two derivatives. The drizzle interpolant returns
+the interpolant value and the first derivative.
+.le
+.ih
+DESCRIPTION
+The polynomial coefficients are evaluated directly from the data points
+for the polynomial interpolants and from the B-spline coefficients
+for the cubic spline interpolant. The derivatives are evaluated from
+the polynomial coefficients using nested multiplication.  The sinc
+derivatives are analytic but are defined only for the first two derivatives.
+The drizzle derivative is an approximation  defined  for the first derivative
+only.
+.ih
+NOTES
+ASIDER checks that the number of derivatives requested is reasonable.
+Checking for out of bounds and INDEF valued pixels is the responsibility
+of the user. ASIINIT or ASISINIT and ASIFIT must be called before ASIDER
+is called.
+.ih
+SEE ALSO
+asieval, asivector, arieval, arider
+.endhelp