On day one, I was fined 2 dollars. Every subsequent day the fine is squared (i.e., day 1: 2, day 2: 4, day 3: 16, day 4: 256, day 5: 65,536). After some analysis, I came up with the formula to determine the fine amount D given the day N: D = $2^{2^{(N-1)}}$.

I am now trying to prove this formula is true using mathematical induction, but I do not know what to show for the k + 1 step. Assume N=k is true: D = $2^{2^{(k-1)}}$

Show k + 1 is true: ???

  • $\begingroup$ Please use MathJax to format your equations. $\endgroup$
    – Gary
    Jan 17 at 2:20
  • 1
    $\begingroup$ Square the 2^(2^(k-1)) and check that it is indeed 2^(2^([k+1]-1)). $\endgroup$ Jan 17 at 2:20

1 Answer 1


To figure out what to do, it helps to clarify two things:

  1. The fine amount $D$ is, first of all, a function of $N$. We can make this easier to keep in mind by writing it as $D_N$ or $D(N)$.
  2. We can phrase the information you're given as $D_1 = 2$ (on day $1$, you are fined $2$ dollars) and $D_N = (D_{N-1})^2$ (every day, the fine is squared).

Now, in the induction hypothesis, rather than saying "$N=k$ is true" which is vague and poorly phrased, we can be more precise. We are assuming that the formula we are trying to prove, that $D_N = 2^{2^{N-1}}$, holds when $N=k$. In other words, $D_k = 2^{2^{k-1}}$.

We are now trying to prove that this formula also holds for $N=k+1$: that $D_{k+1} = 2^{2^{k}}$. Can you prove this using the information you're given - that $D_{k+1} = (D_k)^2$?


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