Edge colourings of an icosahedron
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I'm referring to problem A6 of the 2017 Putnam competition -- the question is "How many ways exist to colour the labelled edges of an icosahedron such that every face has two edges of the same colour and one edge of another colour, where the colours are either red, white or blue?".
My solution is as follows: consider the planar representation of the icosahedron:
Note that:
There are 18 ways to colour the edges of a triangle such that it has two edges of the same colour and one edge of another colour (3 choices for the colour that appears twice, 2 choices for the colour that appears once, and 3 choices for the arrangement).
Given the colouring on any one edge of a triangle, there are 6 ways to colour the remaining edges in a way that satisfies the condition (WLOG suppose the given edge is white -- then either one other edge is white (and the other edge is red or blue), which has 4 possibilities, or both the other edges have the same colour, red or blue, which is possible in 2 ways.
Given the colouring on two edges of a triangle, there are 2 ways to colour the remaining edge in a way that satisfies the condition. WLOG, the given edges are coloured either "R R" or "R B". If it's "R R", the 2 ways to choose the other edge are "W" and "B" -- if it's "R B", the 2 ways to choose the other edge are "R" and "B".
So there are 18 ways to choose the colouring on the central triangle (the base case) of the planar representation, and you can write each the number of ways to colour each successive "containing triangle" as $6^32^3$ multiplied by the smaller triangle it contains, so the number of ways to colour the entire icosahedron should be:
$$18(6^32^3)^3=2^{19}3^{11}$$
Unfortunately, the official solution (p. 5) presents an answer of $2^{20}3^{10}$ -- I'm off by a factor of $2/3$! What's going on? What did I do wrong?
combinatorics coloring planar-graph
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I'm referring to problem A6 of the 2017 Putnam competition -- the question is "How many ways exist to colour the labelled edges of an icosahedron such that every face has two edges of the same colour and one edge of another colour, where the colours are either red, white or blue?".
My solution is as follows: consider the planar representation of the icosahedron:
Note that:
There are 18 ways to colour the edges of a triangle such that it has two edges of the same colour and one edge of another colour (3 choices for the colour that appears twice, 2 choices for the colour that appears once, and 3 choices for the arrangement).
Given the colouring on any one edge of a triangle, there are 6 ways to colour the remaining edges in a way that satisfies the condition (WLOG suppose the given edge is white -- then either one other edge is white (and the other edge is red or blue), which has 4 possibilities, or both the other edges have the same colour, red or blue, which is possible in 2 ways.
Given the colouring on two edges of a triangle, there are 2 ways to colour the remaining edge in a way that satisfies the condition. WLOG, the given edges are coloured either "R R" or "R B". If it's "R R", the 2 ways to choose the other edge are "W" and "B" -- if it's "R B", the 2 ways to choose the other edge are "R" and "B".
So there are 18 ways to choose the colouring on the central triangle (the base case) of the planar representation, and you can write each the number of ways to colour each successive "containing triangle" as $6^32^3$ multiplied by the smaller triangle it contains, so the number of ways to colour the entire icosahedron should be:
$$18(6^32^3)^3=2^{19}3^{11}$$
Unfortunately, the official solution (p. 5) presents an answer of $2^{20}3^{10}$ -- I'm off by a factor of $2/3$! What's going on? What did I do wrong?
combinatorics coloring planar-graph
add a comment |
up vote
4
down vote
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up vote
4
down vote
favorite
I'm referring to problem A6 of the 2017 Putnam competition -- the question is "How many ways exist to colour the labelled edges of an icosahedron such that every face has two edges of the same colour and one edge of another colour, where the colours are either red, white or blue?".
My solution is as follows: consider the planar representation of the icosahedron:
Note that:
There are 18 ways to colour the edges of a triangle such that it has two edges of the same colour and one edge of another colour (3 choices for the colour that appears twice, 2 choices for the colour that appears once, and 3 choices for the arrangement).
Given the colouring on any one edge of a triangle, there are 6 ways to colour the remaining edges in a way that satisfies the condition (WLOG suppose the given edge is white -- then either one other edge is white (and the other edge is red or blue), which has 4 possibilities, or both the other edges have the same colour, red or blue, which is possible in 2 ways.
Given the colouring on two edges of a triangle, there are 2 ways to colour the remaining edge in a way that satisfies the condition. WLOG, the given edges are coloured either "R R" or "R B". If it's "R R", the 2 ways to choose the other edge are "W" and "B" -- if it's "R B", the 2 ways to choose the other edge are "R" and "B".
So there are 18 ways to choose the colouring on the central triangle (the base case) of the planar representation, and you can write each the number of ways to colour each successive "containing triangle" as $6^32^3$ multiplied by the smaller triangle it contains, so the number of ways to colour the entire icosahedron should be:
$$18(6^32^3)^3=2^{19}3^{11}$$
Unfortunately, the official solution (p. 5) presents an answer of $2^{20}3^{10}$ -- I'm off by a factor of $2/3$! What's going on? What did I do wrong?
combinatorics coloring planar-graph
I'm referring to problem A6 of the 2017 Putnam competition -- the question is "How many ways exist to colour the labelled edges of an icosahedron such that every face has two edges of the same colour and one edge of another colour, where the colours are either red, white or blue?".
My solution is as follows: consider the planar representation of the icosahedron:
Note that:
There are 18 ways to colour the edges of a triangle such that it has two edges of the same colour and one edge of another colour (3 choices for the colour that appears twice, 2 choices for the colour that appears once, and 3 choices for the arrangement).
Given the colouring on any one edge of a triangle, there are 6 ways to colour the remaining edges in a way that satisfies the condition (WLOG suppose the given edge is white -- then either one other edge is white (and the other edge is red or blue), which has 4 possibilities, or both the other edges have the same colour, red or blue, which is possible in 2 ways.
Given the colouring on two edges of a triangle, there are 2 ways to colour the remaining edge in a way that satisfies the condition. WLOG, the given edges are coloured either "R R" or "R B". If it's "R R", the 2 ways to choose the other edge are "W" and "B" -- if it's "R B", the 2 ways to choose the other edge are "R" and "B".
So there are 18 ways to choose the colouring on the central triangle (the base case) of the planar representation, and you can write each the number of ways to colour each successive "containing triangle" as $6^32^3$ multiplied by the smaller triangle it contains, so the number of ways to colour the entire icosahedron should be:
$$18(6^32^3)^3=2^{19}3^{11}$$
Unfortunately, the official solution (p. 5) presents an answer of $2^{20}3^{10}$ -- I'm off by a factor of $2/3$! What's going on? What did I do wrong?
combinatorics coloring planar-graph
combinatorics coloring planar-graph
edited 14 hours ago
asked 15 hours ago
Abhimanyu Pallavi Sudhir
803619
803619
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You have not checked whether the last triangle (the infinite outer triangle in the figure) is colored correctly. The rest seems o.k. to me.
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1 Answer
1
active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
0
down vote
You have not checked whether the last triangle (the infinite outer triangle in the figure) is colored correctly. The rest seems o.k. to me.
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0
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You have not checked whether the last triangle (the infinite outer triangle in the figure) is colored correctly. The rest seems o.k. to me.
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up vote
0
down vote
up vote
0
down vote
You have not checked whether the last triangle (the infinite outer triangle in the figure) is colored correctly. The rest seems o.k. to me.
You have not checked whether the last triangle (the infinite outer triangle in the figure) is colored correctly. The rest seems o.k. to me.
answered 8 hours ago
Christian Blatter
170k7111324
170k7111324
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