MAT 2143, Winter 2022
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MAT 2143, Winter 2022
1. Recall that the dihedral group Dn is the group of symmetries of a regular n-gon. (a) [3 points] Prove that if n is even then Dn has n + 1 elements of order 2. Hint: You can start by investigating Dn for a small value of n, say n = 2 or n = 4.
(b) [2 points] What is the answer to part (a) in case n is odd? You should justify your answer.
2. Recall that sn denotes the group of permutations on {1.....n}.
(a) [1 point] Does s17 have a subgroup of order 38? You should justify your answer.
Hint: You can use the fact that there are n! permutations in Sn .
(b) [2 points] Let n e N. Prove that for every integer d < n, the group sn has a subgroup with d elements.
Hint. Try to give an explicit example of such a subgroup.
3. The Fif≠eeη Pa之之1e was invented in late 19th century, and has survived until today. The puzzle consists of a 4 x 4 grid, filled by 15 tiles numbered 1,2,. . . ,15. The sixteenth square in the grid is left vacant to allow the tiles to move. A valid movement is to slide a numbered tile to the vacant square.
In this question, you are going to find an explanation for the following fact: with valid moves we are unable to switch 14 and 15 in the original (sorted) arrangement of the tiles. See Figure 1 below:
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Your goal is to prove that it is impossible to go from the left arrangement to the right arrangement using valid moves.
Figure 1.
Here is the proof strategy: w1e assign a permutation in the group s15 to any arrangement of
the tiles as follows: we follow the dotted line from a to B and write the numbers on the tiles in a sequence from left to right, ignoring the vacant square. This sequence will determine the second row in the array notation for a permutation σ e s15 .
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For example, the permutation obtained from the arrangement
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15 |
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14 |
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9 |
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is σ = ╱ 112 |
2 15 |
3 11 |
4 8 |
5 3 |
6 6 |
7 7 |
8 14 |
9 2 |
10 5 |
11 4 |
12 13 |
13 10 |
14 9 |
115 、 |
(a) [1 point] Suppose that we move a tile horizontally. Does σ change? You should justify your answer.
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An example of a horizontal move.
(b) [2 points] Assume that we move a tile vertically. How does σ change? Describe the relation between the two σ’s before and after the move precisely.
Hint: Can you obtain the new σ as the product of the old σ and some transpositions?
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An example of a vertical move.
(c) [2 points] Using parts (a) and (b), and other facts about permutations that you learned in class, prove that the transition in Figure 1 is not possible by a sequence of valid moves.
2022-03-16