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tfp.math.ode.BDF

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Class BDF

Backward differentiation formula (BDF) solver for stiff ODEs.

Inherits From: Solver

Implements the solver described in [Shampine and Reichelt (1997)][1], a variable step size, variable order (VSVO) BDF integrator with order varying between 1 and 5.

Algorithm details

Each step involves solving the following nonlinear equation by Newton's method:

0 = 1/1 * BDF(1, y)[n+1] + ... + 1/k * BDF(k, y)[n+1]
  - h ode_fn(t[n+1], y[n+1])
  - bdf_coefficients[k-1] * (1/1 + ... + 1/k) * (y[n+1] - y[n] - BDF(1, y)[n]
                                                        -  ... - BDF(k, y)[n])

where k >= 1 is the current order of the integrator, h is the current step size, bdf_coefficients is a list of numbers that parameterizes the method, and BDF(m, y) is the m-th order backward difference of the vector y. In particular, BDF(0, y)[n] = y[n] and BDF(m + 1, y)[n] = BDF(m, y)[n] - BDF(m, y)[n - 1] for m >= 0.

Newton's method can fail because * the method has exceeded the maximum number of iterations, * the method is converging too slowly, or * the method is not expected to converge (the last two conditions are determined by approximating the Lipschitz constant associated with the iteration).

When evaluate_jacobian_lazily is True, the solver avoids evaluating the Jacobian of the dynamics function as much as possible. In particular, Newton's method will try to use the Jacobian from a previous integration step. If Newton's method fails with an out-of-date Jacobian, the Jacobian is re-evaluated and Newton's method is restarted. If Newton's method fails and the Jacobian is already up-to-date, then the step size is decreased and Newton's method is restarted.

Even if Newton's method converges, the solution it generates can still be rejected if it exceeds the specified error tolerance due to truncation error. In this case, the step size is decreased and Newton's method is restarted.

References

[1]: Lawrence F. Shampine and Mark W. Reichelt. The MATLAB ODE Suite. SIAM Journal on Scientific Computing 18(1):1-22, 1997.

__init__

View source

__init__(
    rtol=0.001,
    atol=1e-06,
    first_step_size=None,
    safety_factor=0.9,
    min_step_size_factor=0.1,
    max_step_size_factor=10.0,
    max_num_steps=None,
    max_order=bdf_util.MAX_ORDER,
    max_num_newton_iters=4,
    newton_tol_factor=0.1,
    newton_step_size_factor=0.5,
    bdf_coefficients=(-0.185, -1.0 / 9.0, -0.0823, -0.0415, 0.0),
    evaluate_jacobian_lazily=False,
    use_pfor_to_compute_jacobian=True,
    validate_args=False,
    name='bdf'
)

Initializes the solver.

Args:

  • rtol: Optional float Tensor specifying an upper bound on relative error, per element of the dependent variable. The error tolerance for the next step is tol = atol + rtol * abs(state) where state is the computed state at the current step (see also atol). The next step is rejected if it incurs a local truncation error larger than tol. Default value: 1e-3.
  • atol: Optional float Tensor specifying an upper bound on absolute error, per element of the dependent variable (see also rtol). Default value: 1e-6.
  • first_step_size: Optional scalar float Tensor specifying the size of the first step. If unspecified, the size is chosen automatically. Default value: None.
  • safety_factor: Scalar positive float Tensor. When Newton's method converges, the solver may choose to update the step size by applying a multiplicative factor to the current step size. This factor is factor = clamp(factor_unclamped, min_step_size_factor, max_step_size_factor) where factor_unclamped = error_ratio**(-1. / (order + 1)) * safety_factor (see also min_step_size_factor and max_step_size_factor). A small (respectively, large) value for the safety factor causes the solver to take smaller (respectively, larger) step sizes. A value larger than one, though not explicitly prohibited, is discouraged. Default value: 0.9.
  • min_step_size_factor: Scalar float Tensor (see safety_factor). Default value: 0.1.
  • max_step_size_factor: Scalar float Tensor (see safety_factor). Default value: 10..
  • max_num_steps: Optional scalar integer Tensor specifying the maximum number of steps allowed (including rejected steps). If unspecified, there is no upper bound on the number of steps. Default value: None.
  • max_order: Scalar integer Tensor taking values between 1 and 5 (inclusive) specifying the maximum BDF order. Default value: 5.
  • max_num_newton_iters: Optional scalar integer Tensor specifying the maximum number of iterations per invocation of Newton's method. If unspecified, there is no upper bound on the number iterations. Default value: 4.
  • newton_tol_factor: Scalar float Tensor used to determine the stopping condition for Newton's method. In particular, Newton's method terminates when the distance to the root is estimated to be less than newton_tol_factor * norm(atol + rtol * abs(state)) where state is the computed state at the current step. Default value: 0.1.
  • newton_step_size_factor: Scalar float Tensor specifying a multiplicative factor applied to the size of the integration step when Newton's method fails. Default value: 0.5.
  • bdf_coefficients: 1-D float Tensor with 5 entries that parameterize the solver. The default values are those proposed in [1]. Default value: (-0.1850, -1. / 9., -0.0823, -0.0415, 0.).
  • evaluate_jacobian_lazily: Optional boolean specifying whether the Jacobian should be evaluated at each point in time or as needed (i.e., lazily). Default value: True.
  • use_pfor_to_compute_jacobian: Boolean specifying whether or not to use parallel for in computing the Jacobian when jacobian_fn is not specified. Default value: True.
  • validate_args: Whether to validate input with asserts. If validate_args is False and the inputs are invalid, correct behavior is not guaranteed. Default value: False.
  • name: Python str name prefixed to Ops created by this function. Default value: None (i.e., 'bdf').

Methods

solve

View source

solve(
    ode_fn,
    initial_time,
    initial_state,
    solution_times,
    jacobian_fn=None,
    jacobian_sparsity=None,
    batch_ndims=None,
    previous_solver_internal_state=None
)

Solves an initial value problem.

An initial value problem consists of a system of ODEs and an initial condition:

dy/dt(t) = ode_fn(t, y(t))
y(initial_time) = initial_state

Here, t (also called time) is a scalar float Tensor and y(t) (also called the state at time t) is an N-D float or complex Tensor.

Example

The ODE dy/dt(t) = dot(A, y(t)) is solved below.

t_init, t0, t1 = 0., 0.5, 1.
y_init = tf.constant([1., 1.], dtype=tf.float64)
A = [[-1., -2.], [-3., -4.]]

def ode_fn(t, y):
  return tf.linalg.matvec(A, y)

results = tfp.math.ode.BDF.solve(ode_fn, t_init, y_init,
                                 solution_times=[t0, t1])
y0 = results.states[0]  # == dot(matrix_exp(A * t0), y_init)
y1 = results.states[1]  # == dot(matrix_exp(A * t1), y_init)

Using instead solution_times=tfp.math.ode.ChosenBySolver(final_time=1.) yields the state at various times between t_init and final_time chosen automatically by the solver. In this case, results.states[i] is the state at time results.times[i].

Gradient

The gradient of the result is computed using the adjoint sensitivity method described in [Chen et al. (2018)][1].

grad = tf.gradients(y1, y0) # == dot(e, J)
# J is the Jacobian of y1 with respect to y0. In this case, J = exp(A * t1).
# e = [1, ..., 1] is the row vector of ones.

References

[1]: Chen, Tian Qi, et al. "Neural ordinary differential equations." Advances in Neural Information Processing Systems. 2018.

Args:

  • ode_fn: Function of the form ode_fn(t, y). The input t is a scalar float Tensor. The input y and output are both Tensors with the same shape and dtype as initial_state.
  • initial_time: Scalar float Tensor specifying the initial time.
  • initial_state: N-D float or complex Tensor specifying the initial state. The dtype of initial_state must be complex for problems with complex-valued states (even if the initial state is real).
  • solution_times: 1-D float Tensor specifying a list of times. The solver stores the computed state at each of these times in the returned Results object. Must satisfy initial_time <= solution_times[0] and solution_times[i] < solution_times[i+1]. Alternatively, the user can pass tfp.math.ode.ChosenBySolver(final_time) where final_time is a scalar float Tensor satisfying initial_time < final_time. Doing so requests that the solver automatically choose suitable times up to and including final_time at which to store the computed state.
  • jacobian_fn: Optional function of the form jacobian_fn(t, y). The input t is a scalar float Tensor. The input y has the same shape and dtype as initial_state. The output is a 2N-D Tensor whose shape is initial_state.shape + initial_state.shape and whose dtype is the same as initial_state. In particular, the (i1, ..., iN, j1, ..., jN)-th entry of jacobian_fn(t, y) is the derivative of the (i1, ..., iN)-th entry of ode_fn(t, y) with respect to the (j1, ..., jN)-th entry of y. If this argument is left unspecified, the solver automatically computes the Jacobian if and when it is needed. Default value: None.
  • jacobian_sparsity: Optional 2N-D boolean Tensor whose shape is initial_state.shape + initial_state.shape specifying the sparsity pattern of the Jacobian. This argument is ignored if jacobian_fn is specified. Default value: None.
  • batch_ndims: Optional nonnegative integer. When specified, the first batch_ndims dimensions of initial_state are batch dimensions. Default value: None.
  • previous_solver_internal_state: Optional solver-specific argument used to warm-start this invocation of solve. Default value: None.

Returns:

Object of type Results.