Prove using induction that $2^{4^n}+5$ is divisible by 21 I have to show, using induction, that $2^{4^n}+5$ is divisible by $21$. It is supposed to be a standard exercise, but no matter what I try, I get to a point where I have to use two more inductions.
For example, here is one of the things I tried:
Assuming that $21 |2^{4^k}+5$, we have to show that $21 |2^{4^{k+1}}+5$.
Now, $2^{4^{k+1}}+5=2^{4\cdot 4^k}+5=2^{4^k+3\cdot 4^k}+5=2^{4^k}2^{3\cdot 4^k}+5=2^{4^k}2^{3\cdot 4^k}+5+5\cdot 2^{3\cdot 4^k}-5\cdot 2^{3\cdot 4^k}=2^{3\cdot 4^k}(2^{4^k}+5)+5(1-2^{3\cdot 4^k})$.
At this point, the only way out (as I see it) is to prove (using another induction) that $21|5(1-2^{3\cdot 4^k})$. But when I do that, I get another term of this sort, and another induction.
I also tried proving separately that $3 |2^{4^k}+5$ and $7 |2^{4^k}+5$. The former is OK, but the latter is again a double induction.
Is there an easier way of doing this?
Thank you!
EDIT
By an "easier way" I still mean a way using induction, but only once (or at most twice). Maybe add and subtract something different than what I did?...
Just to put it all in a context: a daughter of a friend got this exercise in her very first HW assignment, after a lecture about induction which included only the most basic examples. I tried helping her, but I can't think of a solution suitable for this stage of the course. That's why I thought that there should be a trick I am missing... 

You may have to use a few tricks here. The base case is clear.
Note that $2^{(4^n)} - 2^{(4^{n-1})} = 2^{(4^{n-1})}(2^{(3 \cdot 4^{n-1})} -1)$
So we have to prove that $2^{3  \cdot 4^{n-1}} - 1$ is a multiple of $21$. To do this, note that for $n \geq 2$,we can use  modular arithmetic: since $3  \cdot 4^{n-1}$ is even, $3$ divides $(2^{3  \cdot 4^{n-1}} - 1)$ by Fermat's theorem. Similarly, since $3  \cdot 4^{n-1}$ is also a multiple of $6$, again by Fermat's theorem, $7$ divides $2^{3  \cdot 4^{n-1}} - 1$. Hence, $21$ divides it also.
Now, note that $2^{(4^n)} + 5 = 2^{(4^{n-1})} + 5 + (2^{(4^n)} - 2^{(4^{n-1})})$, hence assuming induction hypothesis, we have the result.
