0
Triangular, 10 bytes
$\:_%i/2*<
Ungolfed:
$
\ :
_ % i
/ 2 * <
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The code, without directionals, is read as $:i*2_%.
$ reads an integer x, stack contains {x}.
: duplicates it, stack contains {x,x}.
i increments the top of stack, stack contains {x,x+1}.
* multiplies the ...