Unambiguous CFG for $a^ib^j$ where $i \le j \le 2i$ could you please help me for finding an unambiguous CFG for the following expression:
$a^ib^j$ where $i \le j \le 2i$
$S \rightarrow$ $aSb$  |   $J$

$J \rightarrow aJbb$ | $ε$

This has only one derivation tree for a word in that language.