# A blind spot in the literature¶

**Memory usage, performances.**
Out of the box, the tensor-centric interfaces of PyTorch,
TensorFlow, Matlab and NumPy strike a **good balance between
power and simplicity**: explicit matrices allow users to implement
standard engineering tools with a code that stays close to the maths.

But there is a limit to what full matrices can handle: **whenever the
operators involved present some structure, baseline matrix-vector
products can be vastly outperformed by domain-specific
implementations.** Some examples of this rule are well-known and
supported by major frameworks through dedicated methods and “layers”:

In Image Processing,

**convolutions**,**Fourier**and**Wavelet transforms**rely on ad hoc schemes that do not involve circulant or Vandermonde matrices.On graph or mesh data,

**sparse matrices**are encoded as lists of indices plus coefficients and provide support for local operators: graph Laplacians, divergences, etc.

**KeOps: adding support for symbolic tensors.**
Surprisingly, though, little to no effort has been made to support
generic **mathematical** or **“symbolic” matrices**, which are not
*sparse* in the traditional sense but can nevertheless be described
compactly in memory using a *symbolic formula* and some small data
arrays.

Allowing the users of **kernel or distance matrices** to bypass the
*transfer* and *storage* of superfluous quadratic buffers is the main
purpose of the KeOps library. As a bite-sized example of our
interface, the program below is a revision of the script presented
in the previous section that **scales up to clouds of** \(\mathrm{N}\,=\,1,000,000\)
**samples in less than a second on modern hardware**,
with a linear memory footprint – remark the
absence of any \(\mathrm{N}\)-by-\(\mathrm{N}\) buffer in the graph.

```
from pykeops.torch import LazyTensor # Semi-symbolic wrapper for torch Tensors
q_i = LazyTensor( q[:,None,:] ) # (N,D) Tensor -> (N,1,D) Symbolic Tensor
q_j = LazyTensor( q[None,:,:] ) # (N,D) Tensor -> (1,N,D) Symbolic Tensor
D_ij = ((q_i - q_j) ** 2).sum(dim=2) # Symbolic matrix of squared distances
K_ij = (- D_ij / (2 * s**2) ).exp() # Symbolic Gaussian kernel matrix
v = K_ij@p # Genuine torch Tensor. (N,N)@(N,D) = (N,D)
# Finally, compute the kernel norm H(q,p):
H = .5 * torch.dot( p.view(-1), v.view(-1) ) # .5 * <p,v>
# Display the computational graph in the figure below, annotated by hand:
make_dot(H, {'q':q, 'p':p}).render(view=True)
```