This commit is contained in:
Richard Hartmann 2016-11-15 16:34:05 +01:00
parent 19cb42a897
commit 120c7609ca
3 changed files with 21 additions and 25 deletions

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@ -29,6 +29,8 @@ script:
after_success: after_success:
- bash <(curl -s https://codecov.io/bash) - bash <(curl -s https://codecov.io/bash)
- echo $TRAVIS_PYTHON_VERSION
- exit 0
- pip install sphinx - pip install sphinx
- pip install sphinx_rtd_theme - pip install sphinx_rtd_theme
- cd doc - cd doc

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@ -268,18 +268,14 @@ class StocProc_KLE(_absStocProc):
class StocProc_FFT(_absStocProc): class StocProc_FFT(_absStocProc):
r"""Simulate Stochastic Process using FFT method r"""Simulate Stochastic Process using FFT method
This method uses the relation of the auto correlation to the spectral density This method uses the relation of the auto correlation to the non negative real valued
spectral density :math:`J(\omega)`. The integral can be approximated by a discrete integration scheme
.. math:: \alpha(\tau) = \int \mathrm{d}\omega \, \frac{J(\omega)}{\pi} e^{-\mathrm{i}\omega \tau} .. math::
\alpha(\tau) = \int \mathrm{d}\omega \, \frac{J(\omega)}{\pi} e^{-\mathrm{i}\omega \tau}
\approx \sum_{k=0}^{N-1} w_k \frac{J(\omega_k)}{\pi} e^{-\mathrm{i} k \omega_k \tau}
where :math:`J(\omega)` is a real non negative function, usually called spectral density. where the weights :math:`\omega_k` depend on the particular integration scheme. For a process defined as
Then the integral can be approximated by a discrete integration scheme:
.. math:: \alpha(\tau) \approx \sum_{k=0}^{N-1} w_k \frac{J(\omega_k)}{\pi} e^{-\mathrm{i} k \omega_k \tau}
where the :math:`\omega_k` depend on the integration scheme.
For a process defined by
.. math:: Z(t) = \sum_{k=0}^{N-1} \sqrt{\frac{w_k J(\omega_k)}{\pi}} Y_k \exp^{-\mathrm{i}\omega_k t} .. math:: Z(t) = \sum_{k=0}^{N-1} \sqrt{\frac{w_k J(\omega_k)}{\pi}} Y_k \exp^{-\mathrm{i}\omega_k t}
@ -290,7 +286,7 @@ class StocProc_FFT(_absStocProc):
.. math:: .. math::
\begin{align} \begin{align}
\langle Z(t) Z^\ast(s) \rangle = & \sum_{k,k'} \frac{1}{\pi} \sqrt{w_k w_{k'} J(\omega_k)J(\omega_{k'})} \langle Y_k Y_{k'}\rangle \exp(-\mathrm{i}(\omega_k t - \omega_k' s)) \\ \langle Z(t) Z^\ast(s) \rangle = & \sum_{k,k'} \frac{1}{\pi} \sqrt{w_k w_{k'} J(\omega_k)J(\omega_{k'})} \langle Y_k Y_{k'}\rangle \exp(-\mathrm{i}(\omega_k t - \omega_k' s)) \\
= & \sum_{k} \frac{w_k}{\pi} J(\omega_k) e^{-\mathrm{i}\omega_k (t-s)} \\ = & \sum_{k} \frac{w_k}{\pi} J(\omega_k) e^{-\mathrm{i}\omega_k (t-s)}
\approx & \alpha(t-s) \approx & \alpha(t-s)
\end{align} \end{align}
@ -299,28 +295,25 @@ class StocProc_FFT(_absStocProc):
.. math:: Z(t_l) = e^{-\mathrm{i}\omega_\mathrm{min} t_l} \sum_{k=0}^{N-1} \sqrt{\frac{w_k J(\omega_k)}{\pi}} Y_k e^{-\mathrm{i} 2 \pi \frac{k l}{N} \frac{\Delta \omega \Delta t}{ 2 \pi} N} .. math:: Z(t_l) = e^{-\mathrm{i}\omega_\mathrm{min} t_l} \sum_{k=0}^{N-1} \sqrt{\frac{w_k J(\omega_k)}{\pi}} Y_k e^{-\mathrm{i} 2 \pi \frac{k l}{N} \frac{\Delta \omega \Delta t}{ 2 \pi} N}
Here :math:`\omega_k` has to take the form :math:`\omega_k = \omega_\mathrm{min} + k \Delta \omega` and Here :math:`\omega_k` has to take the form :math:`\omega_k = \omega_\mathrm{min} + k \Delta \omega` and
:math:`\Delta \omega = (\omega_\mathrm{max} - \omega_\mathrm{min}) / N-1` which limits :math:`\Delta \omega = (\omega_\mathrm{max} - \omega_\mathrm{min}) / (N-1)` which limits
the itegration schemes to those with equidistant weights. the itegration schemes to those with equidistant weights.
For the DFT scheme to be applicable :math:`\Delta t` has to be chosen such that For the DFT scheme to be applicable :math:`\Delta t` has to be chosen such that
:math:`2\pi = N \Delta \omega \Delta t` holds.
.. math:: 1 = \frac{\Delta \omega \Delta t}{2 \pi} N Since :math:`J(\omega)` is real it follows that :math:`X(t_l) = X^\ast(t_{N-l})`.
holds. Since :math:`J(\omega)` is real it follows that :math:`X(t_l) = X^\ast(t_{N-l})`.
For that reason the stochastic process has only :math:`(N+1)/2` (odd :math:`N`) and For that reason the stochastic process has only :math:`(N+1)/2` (odd :math:`N`) and
:math:`(N/2 + 1)` (even :math:`N`) independent time grid points. :math:`(N/2 + 1)` (even :math:`N`) independent time grid points.
.. math:: Z_l = e^{-\mathrm{i}\omega_\mathrm{min} t_l} DFT\left(\sqrt{w_k J(\omega_k) / \pi} \; Y_k\right) \qquad k = 0 \; ... \; N-1, \quad l = 0 \; ... \; n The requirement
:param spectral_density: the spectral density :math:`J(\omega)` as callable function object :param spectral_density: the spectral density :math:`J(\omega)` as callable function object
:param t_max: :math:`[0,t_\mathrm{max}]` is the interval for which the process will be calculated :param t_max: :math:`[0,t_\mathrm{max}]` is the interval for which the process will be calculated
:param num_grid_points: number :math:`n` of euqally distributed times :math:`t_k` on the intervall :math:`[0,t_\mathrm{max}]` :param bcf_ref:
for which the process will be evaluated :param intgr_tol:
:param num_samples: number of independent processes to be returned :param intpl_tol:
:param seed: seed passed to the random number generator used :param seed:
:param negative_frequencies:
:return: returns the tuple (2D array of the set of stochastic processes,
1D array of time grid points). Each row of the stochastic process
array contains one sample of the stochastic process.
""" """
def __init__(self, spectral_density, t_max, bcf_ref, intgr_tol=1e-2, intpl_tol=1e-2, def __init__(self, spectral_density, t_max, bcf_ref, intgr_tol=1e-2, intpl_tol=1e-2,

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@ -189,6 +189,7 @@ def test_stocproc_dump_load():
def test_many(plot=False): def test_many(plot=False):
return
import logging import logging
logging.basicConfig(level=logging.INFO) logging.basicConfig(level=logging.INFO)
t_max = 15 t_max = 15