Speaking of line noise...

(from [wikipedia])

(~R∈R○.×R)/R←↓⍳R

⍳ R :

1, 2, 3, 4 … R

↓ … : drop the first element.

2, 3, 4 ... R

R ← …: specify that R has the value on the rhs.

R ○ . × R : generate outer product of R by R.

4  6  8 10 ...
6  9 12 15 ...
...