.help asider Dec98 "Image Interpolator Package"
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NAME
asider -- evaluate the interpolant derivatives at x
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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
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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.
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.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
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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.
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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.
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SEE ALSO
asieval, asivector, arieval, arider
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