I'm trying to solve the excersice from Knuth's "Concrete Mathematics":

A Double Tower of Hanoi contains 2n disks of n different sizes, two of
each size. As usual, we're required to move only one disk at a time, 
without putting a larger one over a smaller one.

How many moves does it take to transfer a double tower from one peg to
another, if disks of equal size are indistinguishable from each other?

My solution was like this. Let $n$ be the number of disks of different sizes and $X_n$ - number of moves. The first few solutions are:

$n = 0$ $X_n = 0$

$n = 1$ $X_n = 2$

$n = 2$ $X_n = 6$

$n = 3$ $X_n = 14$

Recurrence is $X_n = 2X_{n-1} + 2$

We can add $2$ to both sides:

$X_n+2 = 2X_{n-1} + 4$

let $Y_n = X_n + 2$, then

$Y_n = 2Y_{n-1}$

$Y_n = 2^n$

$X_n = 2^n - 2$

The problem is that the correct solution is $2^{n+1} - 2$, but I cannot find an error in my approach.

  • 1
    $\begingroup$ Possible duplicate of Solving or approximating recurrence relations for sequences of numbers $\endgroup$ Aug 13, 2017 at 9:51
  • $\begingroup$ We discourage "please check whether my answer is correct" questions because they're unlikely to be useful for anyone else in the future. See here and here. Can you edit your post to ask about a specific conceptual issue you're uncertain about? As a rule of thumb, a good conceptual question should be useful even to someone who isn't looking at the problem you happen to be working on. If you just need someone to check your work, you might seek out a friend, classmate, or teacher. $\endgroup$ Aug 13, 2017 at 9:54

1 Answer 1


Your $Y_0$ is probably wrong. $Y_0$ must be equal to $X_0+2=0+2=2$. Then $Y_n=2^{n+1}$ and hence $X_n=2^{n+1}-2$. Please double check.


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