BUG in Python3 f = CubicSpline(x, y, bc_type='natural')
x = [0, 1, 2]
y = [1, 3, 2]

use bc_type = 'natural' adds the constraints as we described above

f = CubicSpline(x, y, bc_type='natural')
f.c
array([[-0.75, 0.75],
[ 0. , -2.25],
[ 2.75, 0.5 ],
[ 1. , 3. ]])
correct
array([[-0.75, 0.75],
[ 0. , -4.5 ],
[ 2.75, 7.25],
[ 1. , -0.5 ]])
you can find the details in https://pythonnumericalmethods.berkeley.edu/notebooks/chapter17.03-Cubic-Spline-Interpolation.html

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