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"""
This module provides an implementation of Filon's integration formula.
For information about Filon's formula, see e.g.
`Abramowitz and Stegun, Handbook of Mathematical Functions,
section 25 <http://mathworld.wolfram.com/FilonsIntegrationFormula.html>`_ or
Allen and Tildesley, *Computer Simulation of Liquids*, Appendix D :cite:`AllTil87`.
Due to the algorithm the number of time samples must be odd i.e. t = dt * [0, ..., 2n]
for some angular frequency w
integral = f(x) @ (+[b/2, g, b, ..., b, g, b/2] * cos(w x)
+[-a, 0, 0, ..., 0, 0, a] * sin(w x))
or
integral =
+ f[0 ] * (b/2 * cos(w x[0]) - a sin(w x[0]))
+ f[1 ] * (g * cos(w x[1]))
+ f[2 ] * (b * cos(w x[2]))
+ ...
+ f[2n-2] * (b * cos(w x[2n-2]))
+ f[2n-1] * (g * cos(w x[2n-1]))
+ f[2n ] * (b/2 * cos(w x[2n]) - a sin(w x[2n]))
"""
import numpy as np
import numba
from numpy.typing import NDArray
from typing import Tuple
def fourier_cos_filon(f: NDArray[float],
dt: float) -> Tuple[NDArray[float], NDArray[float]]:
r"""Calculates the direct Fourier cosine transform :math:`F(w)` of a
function :math:`f(t)` using Filon's integration method.
Parameters
----------
f
function values as a 2D array. second axis will be transformed.
must contain an odd number of elements along second axis.
dt
spacing of t-axis (:math:`\Delta t`)
Returns
-------
w
w containes values in the interval [0, pi/dt).
length of w is f.shape[1] // 2 + 1.
These frequencies corresponds to the frequencies from an fft.
w == 2*np.pi*np.fft.rfftfreq(f.shape[1], dt)
F
transform of f along second axis.
equivalent to np.fft.rfft(f, axis=1).real
Example
-------
A common use case is
.. code-block:: python
w, F = fourier_cos_filon(f, dt)
"""
if f.ndim != 2:
raise ValueError('f must be 2D and last axis corresponding to integration variable')
if f.shape[1] % 2 == 0: # Filon only works for odd N
raise ValueError('f must contain an odd number of elements along second axis.')
if f.shape[1] < 2: # Time signal must be atleast three long
raise ValueError('f must contain atleast 3 elements along second axis.')
w = np.linspace(0, 2 * np.pi / (2 * dt), f.shape[1])
return w, 2 * filon_2D(f, dt)
@numba.njit(fastmath=True, parallel=True)
def filon_2D(f: NDArray[float], dt: float) -> NDArray:
"""Calculates the fourier transform over the last axis using filons method"""
N_rows, Nt = f.shape
assert Nt % 2 == 1 and Nt > 1 # Filon only works for odd N
dw = np.pi / ((Nt - 1) * dt)
Nw = Nt
filon_wr = np.zeros((Nw, N_rows), dtype=np.float64)
for wi in numba.prange(Nw):
w = wi * dw
filon_wr[wi] = filon_2D_inner(f, dt, w)
filon_wr *= dt
return filon_wr.T.copy()
@numba.njit(fastmath=True)
def filon_2D_inner(f, dt, w):
"""Calculates the transform of 1D f"""
N_rows, Nt = f.shape
alpha, beta, gamma = _alpha_beta_gamma_single(dt * w)
t_arr = np.arange(Nt) * dt
phase = np.cos(w * t_arr) # phase = np.sin(w * t_arr) # for sin transform
phase[::2] *= beta # all evens get multiplied with beta
phase[1:-1:2] *= gamma # all odds get multiplied with gamma
# The enpoints (even index) get an extra factor of 1/2
phase[0] *= 0.5
phase[-1] *= 0.5
# The endpoints must also get an extra term
phase[0] -= alpha * np.sin(w * t_arr[0]) # phase[0] += alpha * np.cos(w * t_arr[0])
phase[-1] += alpha * np.sin(w * t_arr[-1]) # phase[-1] -= alpha * np.cos(w * t_arr[-1])
filon = np.zeros(N_rows, dtype=np.float64)
for r in range(N_rows):
for t in range(Nt):
filon[r] += f[r, t] * phase[t]
return filon
@numba.njit(fastmath=False)
def _alpha_beta_gamma_single(t: float):
# From theta (t), calculate alpha, beta, and gamma
if t == 0:
alpha, beta, gamma = 0.0, 2/3, 4/3
else:
alpha = (t**2 + t * np.sin(t) * np.cos(t) - 2 * np.sin(t)**2) / t**3
beta = 2 * (t * (1 + np.cos(t)**2) - 2 * np.sin(t) * np.cos(t)) / t**3
gamma = 4 * (np.sin(t) - t * np.cos(t)) / t**3
return alpha, beta, gamma
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