outer product

An outer product

$$\begin{array}{rl}{\mathbb{R}}^{n\times 1}\times {\mathbb{R}}^{1\times n}& \to {\mathbb{R}}^{n\times n}\\ a_i{b}_j& \mapsto C_{ij}\end{array}$$

results in a matrix where the $n^{\text{th}}$ row is the $n^{\text{th}}$ row of the first vector multiplied by the $n^{\text{th}}$ element of the second vector:

torch.einsum("i, j -> ij", [torch.arange(4), torch.arange(4)])
tensor([[0, 0, 0, 0],
		[0, 1, 2, 3],
		[0, 2, 4, 6],
		[0, 3, 6, 9]])