Prove $a = 2 \cdot 10^{2016} + 16$ cannot be a perfect square 
Prove the number $a = 2 \cdot 10^{2016} + 16$ cannot be a perfect square.

I tried studying $a \mod b$ for different integers $b$, without succes.

Let $$k^2 = 2\cdot 10^{2016} + 16$$ Then we have  $(k-4)(k+4)=2\cdot 10^{2016}$. Thus we have two factors of $2\cdot 10^{2016}$ with a difference of $8$. 
Now if these factors, say $a$ and $b$, both contain $5$ in their prime factorization, then we have that $|a-b|$ is a multiple of $5$ which is not $8$. Thus let $a=2^k$ and $b= 2^m 5^n$, where $k,m,n$ are non-negative integers. We know that $ab = 2^{m+k}5^{n}= 2^{2017}5^{2016}$. Thus $m+k=2017$ and $n=2016$. Now:
$$a-b = 2^k - 2^m5^n = \pm 8$$
$$2^{k-3} - 2^{m-3}5^n = \pm 1$$
Its easy to see that $k\geq 3$ and $n\geq 3$, since both can't be smaller than $3$. Since $\pm 1$ is not divisible by $2$ we have that either $m=3$ or $k=3$.
Thus our possible solutions for $(m,k,n)$ is $(3,2014,2016)$ and $(2014,3,2016)$, neither of which satisfies the last equation. Hence, we have a contradiction.
