from typing import List


def D0_dd( phi: List[float], s: List[float] ) -> float:
	"""
	Compute the central difference approximation of the second order derivative of phi along the same dimension, as in [3].
	:param phi: List of the 3 upwind-ordered phi values (i.e. [\phi_{i-1}, \phi_i, \phi_{i+1}]).
	:param s: List of the 2 distances along the d dimension between the upwind-ordered phi values (i.e. [s_{i-1}, s_{i+1}]).
	:return: D^0_{dd}\phi_i.
	"""
	return 2.0 / sum( s[:2] ) * ( ( phi[2] - phi[1] ) / s[1] - ( phi[1] - phi[0] ) / s[0] )