A positive integer $N$ has its first, third and fifth digits equal and its second, fourth and sixth digits equal. In other words, when written in the usual decimal system it has the form $xyxyxy$, where $x$ and $y$ are the digits. Show that $N$ cannot be a perfect power, i.e., $N$ cannot equal $a ^ { b }$, where $a$ and $b$ are positive integers with $b > 1$.