1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
|
import numpy as np
from scipy.integrate import ode
from .common import validate_tol, warn_extraneous
from .base import OdeSolver, DenseOutput
class LSODA(OdeSolver):
"""Adams/BDF method with automatic stiffness detection and switching.
This is a wrapper to the Fortran solver from ODEPACK [1]_. It switches
automatically between the nonstiff Adams method and the stiff BDF method.
The method was originally detailed in [2]_.
Parameters
----------
fun : callable
Right-hand side of the system. The calling signature is ``fun(t, y)``.
Here ``t`` is a scalar, and there are two options for the ndarray ``y``:
It can either have shape (n,); then ``fun`` must return array_like with
shape (n,). Alternatively it can have shape (n, k); then ``fun``
must return an array_like with shape (n, k), i.e. each column
corresponds to a single column in ``y``. The choice between the two
options is determined by `vectorized` argument (see below). The
vectorized implementation allows a faster approximation of the Jacobian
by finite differences (required for this solver).
t0 : float
Initial time.
y0 : array_like, shape (n,)
Initial state.
t_bound : float
Boundary time - the integration won't continue beyond it. It also
determines the direction of the integration.
first_step : float or None, optional
Initial step size. Default is ``None`` which means that the algorithm
should choose.
min_step : float, optional
Minimum allowed step size. Default is 0.0, i.e. the step size is not
bounded and determined solely by the solver.
max_step : float, optional
Maximum allowed step size. Default is np.inf, i.e. the step size is not
bounded and determined solely by the solver.
rtol, atol : float and array_like, optional
Relative and absolute tolerances. The solver keeps the local error
estimates less than ``atol + rtol * abs(y)``. Here `rtol` controls a
relative accuracy (number of correct digits). But if a component of `y`
is approximately below `atol`, the error only needs to fall within
the same `atol` threshold, and the number of correct digits is not
guaranteed. If components of y have different scales, it might be
beneficial to set different `atol` values for different components by
passing array_like with shape (n,) for `atol`. Default values are
1e-3 for `rtol` and 1e-6 for `atol`.
jac : None or callable, optional
Jacobian matrix of the right-hand side of the system with respect to
``y``. The Jacobian matrix has shape (n, n) and its element (i, j) is
equal to ``d f_i / d y_j``. The function will be called as
``jac(t, y)``. If None (default), the Jacobian will be
approximated by finite differences. It is generally recommended to
provide the Jacobian rather than relying on a finite-difference
approximation.
lband, uband : int or None
Parameters defining the bandwidth of the Jacobian,
i.e., ``jac[i, j] != 0 only for i - lband <= j <= i + uband``. Setting
these requires your jac routine to return the Jacobian in the packed format:
the returned array must have ``n`` columns and ``uband + lband + 1``
rows in which Jacobian diagonals are written. Specifically
``jac_packed[uband + i - j , j] = jac[i, j]``. The same format is used
in `scipy.linalg.solve_banded` (check for an illustration).
These parameters can be also used with ``jac=None`` to reduce the
number of Jacobian elements estimated by finite differences.
vectorized : bool, optional
Whether `fun` is implemented in a vectorized fashion. A vectorized
implementation offers no advantages for this solver. Default is False.
Attributes
----------
n : int
Number of equations.
status : string
Current status of the solver: 'running', 'finished' or 'failed'.
t_bound : float
Boundary time.
direction : float
Integration direction: +1 or -1.
t : float
Current time.
y : ndarray
Current state.
t_old : float
Previous time. None if no steps were made yet.
nfev : int
Number of evaluations of the right-hand side.
njev : int
Number of evaluations of the Jacobian.
References
----------
.. [1] A. C. Hindmarsh, "ODEPACK, A Systematized Collection of ODE
Solvers," IMACS Transactions on Scientific Computation, Vol 1.,
pp. 55-64, 1983.
.. [2] L. Petzold, "Automatic selection of methods for solving stiff and
nonstiff systems of ordinary differential equations", SIAM Journal
on Scientific and Statistical Computing, Vol. 4, No. 1, pp. 136-148,
1983.
"""
def __init__(self, fun, t0, y0, t_bound, first_step=None, min_step=0.0,
max_step=np.inf, rtol=1e-3, atol=1e-6, jac=None, lband=None,
uband=None, vectorized=False, **extraneous):
warn_extraneous(extraneous)
super(LSODA, self).__init__(fun, t0, y0, t_bound, vectorized)
if first_step is None:
first_step = 0 # LSODA value for automatic selection.
elif first_step <= 0:
raise ValueError("`first_step` must be positive or None.")
if max_step == np.inf:
max_step = 0 # LSODA value for infinity.
elif max_step <= 0:
raise ValueError("`max_step` must be positive.")
if min_step < 0:
raise ValueError("`min_step` must be nonnegative.")
rtol, atol = validate_tol(rtol, atol, self.n)
if jac is None: # No lambda as PEP8 insists.
def jac():
return None
solver = ode(self.fun, jac)
solver.set_integrator('lsoda', rtol=rtol, atol=atol, max_step=max_step,
min_step=min_step, first_step=first_step,
lband=lband, uband=uband)
solver.set_initial_value(y0, t0)
# Inject t_bound into rwork array as needed for itask=5.
solver._integrator.rwork[0] = self.t_bound
solver._integrator.call_args[4] = solver._integrator.rwork
self._lsoda_solver = solver
def _step_impl(self):
solver = self._lsoda_solver
integrator = solver._integrator
# From lsoda.step and lsoda.integrate itask=5 means take a single
# step and do not go past t_bound.
itask = integrator.call_args[2]
integrator.call_args[2] = 5
solver._y, solver.t = integrator.run(
solver.f, solver.jac, solver._y, solver.t,
self.t_bound, solver.f_params, solver.jac_params)
integrator.call_args[2] = itask
if solver.successful():
self.t = solver.t
self.y = solver._y
# From LSODA Fortran source njev is equal to nlu.
self.njev = integrator.iwork[12]
self.nlu = integrator.iwork[12]
return True, None
else:
return False, 'Unexpected istate in LSODA.'
def _dense_output_impl(self):
iwork = self._lsoda_solver._integrator.iwork
rwork = self._lsoda_solver._integrator.rwork
order = iwork[14]
h = rwork[11]
yh = np.reshape(rwork[20:20 + (order + 1) * self.n],
(self.n, order + 1), order='F').copy()
return LsodaDenseOutput(self.t_old, self.t, h, order, yh)
class LsodaDenseOutput(DenseOutput):
def __init__(self, t_old, t, h, order, yh):
super(LsodaDenseOutput, self).__init__(t_old, t)
self.h = h
self.yh = yh
self.p = np.arange(order + 1)
def _call_impl(self, t):
if t.ndim == 0:
x = ((t - self.t) / self.h) ** self.p
else:
x = ((t - self.t) / self.h) ** self.p[:, None]
return np.dot(self.yh, x)
|
