"""
Bessel Process BES
"""
import math
from functools import partial
from multiprocessing import Pool
import numpy as np
from scipy.special import eval_genlaguerre
from aleatory.processes.analytical.brownian_motion import BrownianMotion
from aleatory.processes.base import SPExplicit
from aleatory.stats import ncx
from aleatory.utils.utils import check_positive_integer, get_times, sample_bessel_global
def _sample_bessel_global(T, initial, dim, n):
path = sample_bessel_global(T=T, initial=initial, dim=dim, n=n)
return path
[docs]class BESProcess(SPExplicit):
r"""Bessel process
.. image:: _static/bes_process_drawn.png
A Bessel process :math:`BES^{n}_{0},` for :math:`n\geq 2` integer is a continuous stochastic process
:math:`\{X(t) : t \geq 0\}` characterised as the Euclidian norm of an :math:`n`-dimensional
Brownian motion. That is,
.. math::
X_t = \sqrt{\sum_{i=1}^n (W^i_t)^2}.
More generally, for any :math:`\delta >0`, and :math:`x_0 \geq 0`, a Bessel process of dimension :math:`\delta`
starting at :math:`x_0`, denoted by
.. math::
BES_{{x_0}}^{{\delta}}
can be defined by the following SDE
.. math::
dX_t = \frac{(\delta-1)}{2} \frac{dt}{X_t} + dW_t \ \ \ \ t\in (0,T]
with initial condition :math:`X_0 = x_0\geq 0.`, where
- :math:`\delta` is a positive real
- :math:`W_t` is a standard one-dimensional Brownian Motion.
:param double dim: the dimension of the process :math:`n`
:param double initial: the initial point of the process :math:`x_0`
:param double T: the right hand endpoint of the time interval :math:`[0,T]`
for the process
:param numpy.random.Generator rng: a custom random number generator
"""
def __init__(self, dim=1.0, initial=0.0, T=1.0, rng=None):
super().__init__(T=T, rng=rng, initial=initial)
self.dim = dim
self._brownian_motion = BrownianMotion(T=T, rng=rng)
self.name = f'$BES^{{{self.dim}}}_{{{self.initial}}}$'
self.n = None
self.times = None
def __str__(self):
return "Bessel process with dimension {dim} and starting condition {initial} on [0, {T}].".format(
T=str(self.T), dim=str(self.dim), initial=str(self.initial))
def __repr__(self):
return "BESProcess(dimension={dim}, initial={initial}, T={T})".format(
T=str(self.T), dim=str(self.dim), initial=str(self.initial))
@property
def dim(self):
"""Bessel Process dimension."""
return self._dim
@dim.setter
def dim(self, value):
if value < 0:
raise TypeError("Dimension must be positive")
self._dim = value
@property
def initial(self):
"""Bessel Process initial point."""
return self._initial
@initial.setter
def initial(self, value):
if value < 0:
raise TypeError("Initial point must be positive")
self._initial = value
def _sample_bessel_integer_dim_zero(self, n):
check_positive_integer(n)
self.n = n
self.times = get_times(self.T, n)
brownian_samples = [self._brownian_motion.sample(n) for _ in range(self.dim)]
norm = np.array([np.linalg.norm(coord) for coord in zip(*brownian_samples)])
return norm
def sample(self, n):
if isinstance(self.dim, int) and self.initial == 0:
return self._sample_bessel_integer_dim_zero(n)
else:
return _sample_bessel_global(self.T, self.initial, self.dim, n)
[docs] def simulate(self, n, N):
"""
Simulate paths/trajectories from the instanced stochastic process.
param n: number of steps in each path
param N: number of paths to simulate
return: list with N paths (each one is a numpy array of size n)
"""
self.n = n
self.N = N
self.times = get_times(self.T, n)
if isinstance(self.dim, int) and self.initial == 0:
self.paths = [self.sample(n) for _ in range(N)]
return self.paths
else:
pool = Pool()
initial = self.initial
dim = self.dim
T = self.T
func = partial(_sample_bessel_global, T, initial, dim)
results = pool.map(func, [n] * N)
pool.close()
pool.join()
self.paths = results
return self.paths
def get_marginal(self, t):
marginal = ncx(df=self.dim, nc=self.initial / np.sqrt(t), scale=np.sqrt(t))
return marginal
def _process_expectation(self, times=None):
# TODO: Add the case when times is zero, at the moment this fails because nc required division by t
if times is None:
times = self.times
alpha = (self.dim / 2.0) - 1.0
if np.isscalar(times):
nc = (self.initial ** 2) / times
expectations = np.sqrt(times) * math.sqrt(math.pi / 2.0) * eval_genlaguerre(0.5, alpha,
(-1.0 / 2.0) * nc)
else:
nc = (self.initial ** 2) / times[1:]
expectations = np.sqrt(times[1:]) * math.sqrt(math.pi / 2.0) * eval_genlaguerre(0.5, alpha,
(-1.0 / 2.0) * nc)
expectations = np.insert(expectations, 0, self.initial)
return expectations
def marginal_expectation(self, times=None):
expectations = self._process_expectation(times=times)
return expectations
def _process_variance(self, times=None):
if times is None:
times = self.times
expectations = self._process_expectation(times)
variances = self.dim * times + self.initial ** 2 - expectations ** 2
return variances
def marginal_variance(self, times):
variances = self._process_variance(times=times)
return variances
def _process_stds(self):
stds = np.sqrt(self._process_variance())
return stds
def process_stds(self):
stds = self._process_stds()
return stds