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Indecisive Walk (Posted on 2023-08-17) Difficulty: 3 of 5
Marilyn vos Savant still has her column in Parade magazine, which introduced everyone to the Monty Hall Problem. Here's her latest (well, a repeat from 1995):

A schoolboy has a test coming up and so wants to study for it, but is conflicted, as there's a movie he wants to see playing at the local theater. He starts out toward the theater, but halfway there he has second thoughts and turns back toward home. But again, halfway home he has third thoughts and turns back again, this time toward the theater, but again, halfway there he turns back toward home, and this indecision continues in the same way with the same switching ad infinitum.

What does this lead to as to his location in the long run?

  Submitted by Charlie    
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Solution: (Hide)
Marilyn claims that the boy's asymptotic behavior is to "quiver like a tuning fork" 1/3 of the way from his home to the theater.

This is a false analogy from Zeno's paradox. She claims that "the sum of his forward motion totals 2/3 of the way from his house, but the backward motion totals 1/3 of that distance and is subtracted from the 2/3.

But that is only considering the positions at even numbers of steps. After an odd number, the boy is 2/3 of the way to the theater.

Actually he's not "quivering like a tuning fork". but repeatedly going back and forth the full 1/3 of the home-to-theater distance, between 1/3 of the way and 2/3 of the way.:

A listing of his location after each step. Each line shows the location after walking toward the theater, and each location after the second part of that pair. Position 0 is his home; position 1 is the theater:

clearvars,clc
pos=0;
for i=1:30
  newPos=(pos+1)/2;
  pos=newPos/2;
  fprintf('%10d %17.15f %17.15f\\n',[i newPos pos])
end

     pair               position
      of     after going        after going
    walks    toward movie       toward home

         1 0.500000000000000 0.250000000000000
         2 0.625000000000000 0.312500000000000
         3 0.656250000000000 0.328125000000000
         4 0.664062500000000 0.332031250000000
         5 0.666015625000000 0.333007812500000
         6 0.666503906250000 0.333251953125000
         7 0.666625976562500 0.333312988281250
         8 0.666656494140625 0.333328247070312
         9 0.666664123535156 0.333332061767578
        10 0.666666030883789 0.333333015441895
        11 0.666666507720947 0.333333253860474
        12 0.666666626930237 0.333333313465118
        13 0.666666656732559 0.333333328366280
        14 0.666666664183140 0.333333332091570
        15 0.666666666045785 0.333333333022892
        16 0.666666666511446 0.333333333255723
        17 0.666666666627862 0.333333333313931
        18 0.666666666656965 0.333333333328483
        19 0.666666666664241 0.333333333332121
        20 0.666666666666060 0.333333333333030
        21 0.666666666666515 0.333333333333258
        22 0.666666666666629 0.333333333333314
        23 0.666666666666657 0.333333333333329
        24 0.666666666666664 0.333333333333332
        25 0.666666666666666 0.333333333333333
        26 0.666666666666667 0.333333333333333
        27 0.666666666666667 0.333333333333333
        28 0.666666666666667 0.333333333333333
        29 0.666666666666667 0.333333333333333
        30 0.666666666666667 0.333333333333333

Comments: ( You must be logged in to post comments.)
  Subject Author Date
SolutionSolutionLarry2023-08-17 09:03:41
SolutionSolutionJer2023-08-17 08:35:58
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