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Imagine that you possess a very simple electronic calculator. It has a screen and ten buttons from 0 to 9 to enter natural decimal numbers (positive integers). However, it can perform just two unary operations, and that's all. Either:

  1. it can multiply the input number by 2 (e.g. 50 becomes 100); or
  2. it can delete the last digit of the input number (e.g. 50 becomes 5).

Assuming that you have entered a certain number, be it A. How must you act in order to obtain another specified number, be it B? The problem asks for finding a universal algorithm, so you can obtain any number from any other.

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    $\begingroup$ Are you aware of such a universal algorithm? Also, does the length of the algorithm matter? $\endgroup$
    – bobble
    Dec 17 '20 at 21:00
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    $\begingroup$ There is no universal algorithm. If the starting number is 0, you can't get anywhere other than 0. $\endgroup$
    – user253751
    Dec 17 '20 at 21:14
  • $\begingroup$ Here is a related previous question: Two button calculator. $\endgroup$ Dec 17 '20 at 22:01
  • $\begingroup$ @user253751 The natural number condition means the starting number can't be 0, I think. $\endgroup$
    – ripkoops
    Dec 17 '20 at 22:30
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Given that both A and B are positive integers, the first step is

convert A to the number 1

which is easy:

Multiply by 2 until the highest digit overflows to create a new 1 digit, and erase all the lower digits to just leave a single 1.

The next part

of converting the number 1 to an arbitrary given number B

is hard but can be always done in finitely many steps:

We will be searching for a power of 2 that starts with B. If we find it, we can start from 1, multiply by 2 until we reach that particular power of 2, and erase the last digit until we get B.

The inequality to solve is

$$ B\times 10^m \le 2^n < (B+1)\times 10^m \\ m+\log_{10}{B} \le n \log_{10}{2} < m+\log_{10}(B+1) \\ \text{frac}(\log_{10}{B}) \le \text{frac}(n\log_{10}{2}) < \text{frac}(\log_{10}{(B+1)}) $$

Because $\log_{10}{2}$ is irrational,

the inequality always has an integer solution for $n$. For a number-theoretic argument, choose a sufficiently large prime $p$ so that $k/p$ approximates $\log_{10}{2}$ sufficiently well for some integer $k$. If $m/p$ is inside the given interval, $n=mk^{-1} \text{ mod } p$ is a solution. (I forgot the relevant theorem's details and I'm not sure if this number-theoretic argument is valid, but I'm pretty sure the result still holds.)

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