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MU (Posted on 2002-06-20) Difficulty: 4 of 5
(from Gödel, Escher, Bach: an Eternal Golden Braid by Douglas R. Hofstadter)

Start with a single sequence of letters: MI

The goal is to apply a series of manipulations to this sequence to produce MU.

The allowable manipulations are:

1. If the sequence ends in I, you can append U

2. If the sequence begins with M, you can duplicate the string after M, e.g. MIII can be replaced by MIIIIII or MIMU can be replaced by MIMUIMU

3. If the sequence contains III, you can replace the III with U, e.g. UIIIM can be replaced by UUM.

4.If the sequence contains UU, you can delete the UU, e.g. UUM can be replaced by M or MIUUI can be replaced by MII.

These rules are all one-way; you can't take a U and replace it with III.

Is it possible to reach MU from MI? If so, what series of steps would you take to get there? If not, why not?

  Submitted by friedlinguini    
Rating: 3.3333 (6 votes)
Solution: (Hide)
It is not possible.

The trick is in the number of I's. Define f(s) as the number of I's in a string s. To start with, f(MI) = 1, and our goal is f(MU) = 0. Note that f(MI) is not divisible by 3, whereas f(MU) is. In order to reach MU, we have to get f(s) to be divisible by 3 for some s in the series of transformations.

Let's see what happens when each o the rules is applied, changing f(s) to f(s'):

1. f(s') = f(s). No change.
2. f(s') = 2f(s). If f(s) is not divisible by 3, neither is f(s').
3. f(s') = f(s) - 3. No help here.
4. f(s') = f(s). Still no change.

Since the number of I's is not divisible by 3 to start with, and none of the transformations can change that, it is not possible to reach MU.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
answerK Sengupta2007-04-01 15:24:03
no solutionquddous behrouzi2002-06-21 09:39:42
re: THESE PROBLEMS ARE ALL TOO GODDAMN EASYquddous behrouzi2002-06-21 09:04:47
THESE PROBLEMS ARE ALL TOO GODDAMN EASYquddous behrouzi2002-06-21 09:03:57
SolutionSolutionJim Lyon2002-06-21 07:46:05
Hints/Tipssmall hintHappy2002-06-21 07:43:17
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