pyriemann.utils.kernel.kernel_euclid¶

pyriemann.utils.kernel.kernel_euclid(X, Y=None, *, Cref=None, reg=1e-10)¶

Euclidean kernel between two sets of matrices.

Euclidean kernel matrix \(\mathbf{K}\) of two sets \(\mathbf{X}\) and \(\mathbf{Y}\) of matrices in \(\mathbb{R}^{n \times m}\) at \(\mathbf{C}_\text{ref}\) is calculated with pairwise inner products:

\[\mathbf{K}_{i,j} = \text{tr}( (\mathbf{X}_i - \mathbf{C}_\text{ref})^T (\mathbf{Y}_j - \mathbf{C}_\text{ref}) )\]

If \(\mathbf{C}_\text{ref}\) is None [1]:

\[\mathbf{K}_{i,j} = \text{tr}(\mathbf{X}_i^T \mathbf{Y}_j)\]

Parameters:

Xndarray, shape (n_matrices_X, n, m): First set of matrices.
YNone | ndarray, shape (n_matrices_Y, n, m), default=None: Second set of matrices. If None, Y is set to X.
CrefNone | ndarray, shape (n, m), default=None: Reference matrix. If None, Cref is defined as null matrix.

Added in version 0.8.
regfloat, default=1e-10: When Y is None, regularization parameter to mitigate numerical errors in kernel matrix estimation, to provide a positive-definite kernel matrix.

Returns:

Kndarray, shape (n_matrices_X, n_matrices_Y): Euclidean kernel matrix between X and Y.