Eigendecomposition of torch LazyTensor

[huguesva]

Hello,
Thanks for the great library!

I am trying to perform the eigendecomposition of a torch LazyTensor. The following is a slight adaptation of the code in https://www.kernel-operations.io/keops/_auto_tutorials/backends/plot_scipy.html
I simply converted the input data to torch to retrieve a pykeops.torch.LazyTensor

import numpy as np
from pykeops.torch import LazyTensor
import pykeops.config
import torch

from scipy.sparse.linalg import aslinearoperator
from scipy.sparse.linalg import eigsh

dtype = "float32"  # No need for double precision here!

N = 10000 if pykeops.config.gpu_available else 1000
t = np.linspace(0, 2 * np.pi, N + 1)[:-1]
x = np.stack((0.4 + 0.4 * (t / 7) * np.cos(t), 0.5 + 0.3 * np.sin(t)), 1)
x = x + 0.01 * np.random.randn(*x.shape)
x = x.astype(dtype)
x = torch.Tensor(x)

sigma = 0.05
x_ = x / sigma
x_i, x_j = LazyTensor(x_[:, None, :]), LazyTensor(x_[None, :, :])
K_xx = (-((x_i - x_j) ** 2).sum(2) / 2).exp()  # Symbolic (N,N) Gaussian kernel matrix

K = aslinearoperator(K_xx)

eigenvalues, eigenvectors = eigsh(K, k=5)  # Largest 5 eigenvalues/vectors

print("Largest eigenvalues:", eigenvalues)
print("Eigenvectors of shape:", eigenvectors.shape)

This does not seem to work and throws an error from scipy/sparse/linalg/_eigen/arpack/arpack.py .

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
     [21] K_xx = (-((x_i - x_j) ** 2).sum(2) / 2).exp()  # Symbolic (N,N) Gaussian kernel matrix
     [23] K = aslinearoperator(K_xx)
---> [25] eigenvalues, eigenvectors = eigsh(K, k=5)  # Largest 5 eigenvalues/vectors
     [27] print("Largest eigenvalues:", eigenvalues)
     [28] print("Eigenvectors of shape:", eigenvectors.shape)

File /venv/lib/python3.11/site-packages/scipy/sparse/linalg/_eigen/arpack/arpack.py:1700, in eigsh(A, k, M, sigma, which, v0, ncv, maxiter, tol, return_eigenvectors, Minv, OPinv, mode)
   [1698] with _ARPACK_LOCK:
   [1699]     while not params.converged:
-> [1700]         params.iterate()
   [1702]     return params.extract(return_eigenvectors)

File /venv/lib/python3.11/site-packages/scipy/sparse/linalg/_eigen/arpack/arpack.py:549, in _SymmetricArpackParams.iterate(self)
    [546] elif self.ido == 1:
    [547]     # compute y = Op*x
    [548]     if self.mode == 1:
--> [549]         self.workd[yslice] = self.OP(self.workd[xslice])
    [550]     elif self.mode == 2:
    [551]         self.workd[xslice] = self.OPb(self.workd[xslice])

File /venv/lib/python3.11/site-packages/scipy/sparse/linalg/_interface.py:236, in LinearOperator.matvec(self, x)
    [233] if x.shape != (N,) and x.shape != (N,1):
...
     [76] @staticmethod
     [77] def view(x, s):
---> [78]     return x.view(s)

TypeError: Tuple must have size 2, but has size 3

If you have some guidance it would be very helpful.

Thanks a lot.

[huguesva]

After discussion with @jeanfeydy this morning (thanks a lot!), one option would be to use the linear_operator library : https://linear-operator.readthedocs.io/en/latest/index.html instead of aslinearoperatorfrom scipy.sparse.linalg.

Sadly, the function to_linear_operator does not seem to support pykeops torch lazy tensors. Maybe there is another way from linear_operator that I am not aware of.

Thanks in advance!