tfp.math.ode.Solver

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Base class for an ODE solver.

name

Methods

solve

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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), **constants)
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. constants is are values that are constant with respect to time. Passing the constants here rather than just closing over them in ode_fn is only necessary if you want gradients with respect to these values.

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 = tf.constant([[-1., -2.], [-3., -4.]], dtype=tf.float64)

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)

If the exact solution times are not important, it can be much more efficient to let the solver choose them using solution_times=tfp.math.ode.ChosenBySolver(final_time=1.). This yields the state at various times between t_init and final_time, in which case results.states[i] is the state at time results.times[i].

Gradients

The gradients are 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.

This is not capable of computing gradients with respect to values closed over by ode_fn, e.g., in the example above:

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

with tf.GradientTape() as tape:
  tape.watch(A)
  results = tfp.math.ode.BDF().solve(ode_fn, t_init, y_init,
                                     solution_times=[t0, t1])
tape.gradient(results.states, A)  # Undefined!

There are two options to get the gradients flowing through these values:

  1. Use tf.Variable for these values.
  2. Pass the values in explicitly using the constants argument:
def ode_fn(t, y, A):
  return tf.linalg.matvec(A, y)

with tf.GradientTape() as tape:
  tape.watch(A)
  results = tfp.math.ode.BDF().solve(ode_fn, t_init, y_init,
                                     solution_times=[t0, t1],
                                     constants={'A': A})
tape.gradient(results.states, A)  # Fine.

By default, this uses the same solver for the augmented ODE. This can be controlled via make_adjoint_solver_fn.

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, **constants). 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. constants is are values that are constant with respect to time. Passing the constants here rather than just closing over them in ode_fn is only necessary if you want gradients with respect to these values.
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.
constants Optional dictionary with string keys and values being (possibly nested) float Tensors. These represent values that are constant with respect to time. Specifying these here allows the adjoint sentitivity method to compute gradients of the results with respect to these values.

Returns
Object of type Results.