tfp.math.pivoted_cholesky

Computes the (partial) pivoted cholesky decomposition of matrix.

tfp.math.pivoted_cholesky(
    matrix, max_rank, diag_rtol=0.001, return_pivoting_order=False, name=None
)

The pivoted Cholesky is a low rank approximation of the Cholesky decomposition of matrix, i.e. as described in [(Harbrecht et al., 2012)][1]. The currently-worst-approximated diagonal element is selected as the pivot at each iteration. This yields from a [B1...Bn, N, N] shaped matrix a [B1...Bn, N, K] shaped rank-K approximation lr such that lr @ lr.T ~= matrix. Note that, unlike the Cholesky decomposition, lr is not triangular even in a rectangular-matrix sense. However, under a permutation it could be made triangular (it has one more zero in each column as you move to the right).

Such a matrix can be useful as a preconditioner for conjugate gradient optimization, i.e. as in [(Wang et al. 2019)][2], as matmuls and solves can be cheaply done via the Woodbury matrix identity, as implemented by tf.linalg.LinearOperatorLowRankUpdate.

matrix Floating point Tensor batch of symmetric, positive definite matrices.
max_rank Scalar int Tensor, the rank at which to truncate the approximation.
diag_rtol Scalar floating point Tensor (same dtype as matrix). If the errors of all diagonal elements of lr @ lr.T are each lower than element * diag_rtol, iteration is permitted to terminate early.
return_pivoting_order If True, return an int Tensor indicating the pivoting order used to produce lr (in addition to lr).
name Optional name for the op.

lr Low rank pivoted Cholesky approximation of matrix.
perm (Optional) pivoting order used to produce lr.

References

[1]: H Harbrecht, M Peters, R Schneider. On the low-rank approximation by the pivoted Cholesky decomposition. Applied numerical mathematics, 62(4):428-440, 2012.

[2]: K. A. Wang et al. Exact Gaussian Processes on a Million Data Points. arXiv preprint arXiv:1903.08114, 2019. https://arxiv.org/abs/1903.08114