# Help on a perfect square.

Consider a question, that xyxyxyxy cannot be a perfect square. How should i tackle this problem. All i use is it must be $0,1 ($mod $3,4)$ and then the math, are there any another beatiful ways because this method does not work everytime.

Your number is $xy \cdot 1010101$. Can you find a factor that cannot be repeated?
• Another approach is to use the divisibility test for $11$ and then some case checking.But your method is definitely more elegant.Although I'm not sure this "works every time",but what does? – rah4927 Apr 23 '14 at 15:30
• @Shobhit: when you have digit patterns, it is often useful to represent them algebraically. Sometimes it would help to replace $xy$ by $(10x+y)$ One general thing from this answer is if you have $1+x+x^2+\dots x^n$ with $n+1=pq$ you can always factor it as $(1+x+x^2+\dots x^{p-1})(1+x^p+x^{2p}+\dots x^{p(q-1)}$. If $p \neq q$ that gives two different factorizations. – Ross Millikan Apr 23 '14 at 16:30