# Name of operations on two vectors

Suppose we have two vectors $x\in \mathbb{R}^n$ and $y\in \mathbb{R}^m$. I could define the mapping $$T: \mathbb{R}^n\times \mathbb{R}^m \rightarrow \mathbb{R}^{n\times m}$$ as follows $$T(x,y) = ( x_i+y_j )_{i,j=1}^{n,m},$$ i.e. $T(x,y)$ is $n\times m$ matrix with elements equal sum of corresponding vectors. If instead of $x_i+y_j$ one considers $x_i \cdot y_j$ then it is matrix multiplication of two vectors ($x\cdot y^T$, where $x,y$ are column vectors). But what about sum?

Is that operation known and studied somewhere?

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It's not unlikely that this comes up in tropical geometry (en.wikipedia.org/wiki/Tropical_geometry), where the "product" is the original addition. –  Tom De Medts Jun 17 '13 at 14:42
"Outer sum"? I think that's what the programming language APL called it. –  Deane Yang Jun 17 '13 at 15:53
I've occasionally heard "tensor sum". –  Noah Stein Jun 17 '13 at 17:28
Thank you Tom for direction. According to personalpages.manchester.ac.uk/staff/Marianne.Johnson/… It is indeed tropical product of two vectors $a \otimes b^T$. –  Fedor Nikitin Jun 17 '13 at 17:52

It is tropical product of vectors $x$ and $y^T$, i.e. $x \otimes y^T$.
I'd write your operation simply as $x e^T + ey^T$, where $e$ is the vector of all ones. I have never encountered this operation directly studied in a research context (my guess is that it's simply "not interesting enough", since it's a small variation to the common theme of a rank-2 matrix), but I have seen several times the related matrix with elements $$T_{ij}= \frac{1}{x_i - y_j},$$ which is known as Cauchy matrix.
There are two vectors of ones $e_m$ and $e_n$ should be above because I assume that $x$ and $y$ are of dimensions. –  Fedor Nikitin Jun 17 '13 at 20:11