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extrema xtools



extrema -- find the extrema in an array of x and y points


nextrema = extrema (x, y, curvature, npts, dx)

real	x[npts], y[npts]	# Input data points and output extrema
real	curvature[npts]		# 2nd deriv. of cubic spline at extrema
int	npts			# Number of input data points
real	dx			# Precision of extrema positions


The input data points are fitted with a cubic interpolation spline. The spline is then searched for points where the first derivative changes sign. The minimum step size of this search is controlled by the parameter dx. The positions of these extrema are returned in the x array, the value of the spline at the extrema are returned in the y array, and the curvature or second derivative of the spline at the extrema are returned in the curvature array. The function returns the number of extrema found.



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