A Cauchy matrix ([Wikipedia article](http://en.wikipedia.org/wiki/Cauchy_matrix)) is a matrix determined by two vectors (arrays of numbers). Given two vectors `x` and `y`, the Cauchy matrix `C` generated by them is defined entry-wise as

```
C[i][j] := 1/(x[i] - y[j])

```

Given two Numpy arrays `x` and `y`, what is an efficient way to generate a Cauchy matrix?