Base change

Returns a number in a different base. The representation of a number $n$ in base $b$ is $left square bracket a subscript 0 comma a subscript 1 comma... comma a subscript k right square bracket$ if

$n equals a subscript 0 b to the power of 0 plus a subscript 1 b to the power of 1 plus a subscript 2 b squared plus midline horizontal ellipsis plus a subscript k b to the power of k$

with $a subscript i less than b$ .