Four Colour Theorem












7












$begingroup$


I want to colour the US (only the states) map with Yellow, Green, Red and Blue. I was wondering what would be the lowest number of states with the colour of Green. We can of course use the other colours as much as we want. Please note that I want to follow the Four Color Theorem rules.



Motivation:
I am studying graph theory and I want to know if there is a way that we could limit the use of the fourth colour as much as possible. This is not a homework problem.



My attempt:
I have tried many variations and can limit it to 6 and it seems like the
minimum possible but there are infinite possibilities to try so I was wondering if there is a simpler method? Thank you in advance.



Clarification:
I am interested in only the mainland of USA. For states like Michigan that are split, I used the same colour for both parts (since they were not connected directly).






graph-theory recreational-mathematics







share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    you would need to agree on a favorite version of the graph. In the actual US, there are islands, states split into disconnected regions, other things forbidden
    $endgroup$
    – Will Jagy
    1 hour ago






  • 1




    $begingroup$
    blog.computationalcomplexity.org/2006/05/… They correctly point out that three colors cannot work, as Nevada has an odd number of neighbors
    $endgroup$
    – Will Jagy
    1 hour ago










  • $begingroup$
    thank you for your suggestion, I made a few clarifications.
    $endgroup$
    – Bor Kari
    1 hour ago
















7












$begingroup$


I want to colour the US (only the states) map with Yellow, Green, Red and Blue. I was wondering what would be the lowest number of states with the colour of Green. We can of course use the other colours as much as we want. Please note that I want to follow the Four Color Theorem rules.



Motivation:
I am studying graph theory and I want to know if there is a way that we could limit the use of the fourth colour as much as possible. This is not a homework problem.



My attempt:
I have tried many variations and can limit it to 6 and it seems like the
minimum possible but there are infinite possibilities to try so I was wondering if there is a simpler method? Thank you in advance.



Clarification:
I am interested in only the mainland of USA. For states like Michigan that are split, I used the same colour for both parts (since they were not connected directly).






graph-theory recreational-mathematics







share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    you would need to agree on a favorite version of the graph. In the actual US, there are islands, states split into disconnected regions, other things forbidden
    $endgroup$
    – Will Jagy
    1 hour ago






  • 1




    $begingroup$
    blog.computationalcomplexity.org/2006/05/… They correctly point out that three colors cannot work, as Nevada has an odd number of neighbors
    $endgroup$
    – Will Jagy
    1 hour ago










  • $begingroup$
    thank you for your suggestion, I made a few clarifications.
    $endgroup$
    – Bor Kari
    1 hour ago














7












7








7


2



$begingroup$


I want to colour the US (only the states) map with Yellow, Green, Red and Blue. I was wondering what would be the lowest number of states with the colour of Green. We can of course use the other colours as much as we want. Please note that I want to follow the Four Color Theorem rules.



Motivation:
I am studying graph theory and I want to know if there is a way that we could limit the use of the fourth colour as much as possible. This is not a homework problem.



My attempt:
I have tried many variations and can limit it to 6 and it seems like the
minimum possible but there are infinite possibilities to try so I was wondering if there is a simpler method? Thank you in advance.



Clarification:
I am interested in only the mainland of USA. For states like Michigan that are split, I used the same colour for both parts (since they were not connected directly).






graph-theory recreational-mathematics







share|cite|improve this question











$endgroup$




I want to colour the US (only the states) map with Yellow, Green, Red and Blue. I was wondering what would be the lowest number of states with the colour of Green. We can of course use the other colours as much as we want. Please note that I want to follow the Four Color Theorem rules.



Motivation:
I am studying graph theory and I want to know if there is a way that we could limit the use of the fourth colour as much as possible. This is not a homework problem.



My attempt:
I have tried many variations and can limit it to 6 and it seems like the
minimum possible but there are infinite possibilities to try so I was wondering if there is a simpler method? Thank you in advance.



Clarification:
I am interested in only the mainland of USA. For states like Michigan that are split, I used the same colour for both parts (since they were not connected directly).






graph-theory recreational-mathematics




graph-theory recreational-mathematics






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited 1 hour ago







Bor Kari

















asked 1 hour ago









Bor KariBor Kari

3749




3749








  • 1




    $begingroup$
    you would need to agree on a favorite version of the graph. In the actual US, there are islands, states split into disconnected regions, other things forbidden
    $endgroup$
    – Will Jagy
    1 hour ago






  • 1




    $begingroup$
    blog.computationalcomplexity.org/2006/05/… They correctly point out that three colors cannot work, as Nevada has an odd number of neighbors
    $endgroup$
    – Will Jagy
    1 hour ago










  • $begingroup$
    thank you for your suggestion, I made a few clarifications.
    $endgroup$
    – Bor Kari
    1 hour ago














  • 1




    $begingroup$
    you would need to agree on a favorite version of the graph. In the actual US, there are islands, states split into disconnected regions, other things forbidden
    $endgroup$
    – Will Jagy
    1 hour ago






  • 1




    $begingroup$
    blog.computationalcomplexity.org/2006/05/… They correctly point out that three colors cannot work, as Nevada has an odd number of neighbors
    $endgroup$
    – Will Jagy
    1 hour ago










  • $begingroup$
    thank you for your suggestion, I made a few clarifications.
    $endgroup$
    – Bor Kari
    1 hour ago








1




1




$begingroup$
you would need to agree on a favorite version of the graph. In the actual US, there are islands, states split into disconnected regions, other things forbidden
$endgroup$
– Will Jagy
1 hour ago




$begingroup$
you would need to agree on a favorite version of the graph. In the actual US, there are islands, states split into disconnected regions, other things forbidden
$endgroup$
– Will Jagy
1 hour ago




1




1




$begingroup$
blog.computationalcomplexity.org/2006/05/… They correctly point out that three colors cannot work, as Nevada has an odd number of neighbors
$endgroup$
– Will Jagy
1 hour ago




$begingroup$
blog.computationalcomplexity.org/2006/05/… They correctly point out that three colors cannot work, as Nevada has an odd number of neighbors
$endgroup$
– Will Jagy
1 hour ago












$begingroup$
thank you for your suggestion, I made a few clarifications.
$endgroup$
– Bor Kari
1 hour ago




$begingroup$
thank you for your suggestion, I made a few clarifications.
$endgroup$
– Bor Kari
1 hour ago










1 Answer
1






active

oldest

votes


















4












$begingroup$

The minimum is two states that use the fourth color. Nevada and its five neighbors cannot be colored with only three colors, and similarly West Virginia and its five neighbors cannot be colored with only three colors.



But if we color Arizona and Ohio a color we use nowhere else, then the remainder of the map can be completed using only three colors:



enter image description here



Adjacencies between the states may be easier to see here.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
    $endgroup$
    – Will Jagy
    54 mins ago










  • $begingroup$
    @WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
    $endgroup$
    – Misha Lavrov
    53 mins ago










  • $begingroup$
    That's pretty good. A simple standard: at least one drivable road between neighbors
    $endgroup$
    – Will Jagy
    49 mins ago










  • $begingroup$
    just curious -- did you write code to do this, or did you do this by hand?
    $endgroup$
    – antkam
    19 mins ago












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1 Answer
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1 Answer
1






active

oldest

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active

oldest

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active

oldest

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4












$begingroup$

The minimum is two states that use the fourth color. Nevada and its five neighbors cannot be colored with only three colors, and similarly West Virginia and its five neighbors cannot be colored with only three colors.



But if we color Arizona and Ohio a color we use nowhere else, then the remainder of the map can be completed using only three colors:



enter image description here



Adjacencies between the states may be easier to see here.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
    $endgroup$
    – Will Jagy
    54 mins ago










  • $begingroup$
    @WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
    $endgroup$
    – Misha Lavrov
    53 mins ago










  • $begingroup$
    That's pretty good. A simple standard: at least one drivable road between neighbors
    $endgroup$
    – Will Jagy
    49 mins ago










  • $begingroup$
    just curious -- did you write code to do this, or did you do this by hand?
    $endgroup$
    – antkam
    19 mins ago
















4












$begingroup$

The minimum is two states that use the fourth color. Nevada and its five neighbors cannot be colored with only three colors, and similarly West Virginia and its five neighbors cannot be colored with only three colors.



But if we color Arizona and Ohio a color we use nowhere else, then the remainder of the map can be completed using only three colors:



enter image description here



Adjacencies between the states may be easier to see here.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
    $endgroup$
    – Will Jagy
    54 mins ago










  • $begingroup$
    @WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
    $endgroup$
    – Misha Lavrov
    53 mins ago










  • $begingroup$
    That's pretty good. A simple standard: at least one drivable road between neighbors
    $endgroup$
    – Will Jagy
    49 mins ago










  • $begingroup$
    just curious -- did you write code to do this, or did you do this by hand?
    $endgroup$
    – antkam
    19 mins ago














4












4








4





$begingroup$

The minimum is two states that use the fourth color. Nevada and its five neighbors cannot be colored with only three colors, and similarly West Virginia and its five neighbors cannot be colored with only three colors.



But if we color Arizona and Ohio a color we use nowhere else, then the remainder of the map can be completed using only three colors:



enter image description here



Adjacencies between the states may be easier to see here.






share|cite|improve this answer











$endgroup$



The minimum is two states that use the fourth color. Nevada and its five neighbors cannot be colored with only three colors, and similarly West Virginia and its five neighbors cannot be colored with only three colors.



But if we color Arizona and Ohio a color we use nowhere else, then the remainder of the map can be completed using only three colors:



enter image description here



Adjacencies between the states may be easier to see here.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited 45 mins ago

























answered 1 hour ago









Misha LavrovMisha Lavrov

49.3k757108




49.3k757108












  • $begingroup$
    I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
    $endgroup$
    – Will Jagy
    54 mins ago










  • $begingroup$
    @WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
    $endgroup$
    – Misha Lavrov
    53 mins ago










  • $begingroup$
    That's pretty good. A simple standard: at least one drivable road between neighbors
    $endgroup$
    – Will Jagy
    49 mins ago










  • $begingroup$
    just curious -- did you write code to do this, or did you do this by hand?
    $endgroup$
    – antkam
    19 mins ago


















  • $begingroup$
    I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
    $endgroup$
    – Will Jagy
    54 mins ago










  • $begingroup$
    @WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
    $endgroup$
    – Misha Lavrov
    53 mins ago










  • $begingroup$
    That's pretty good. A simple standard: at least one drivable road between neighbors
    $endgroup$
    – Will Jagy
    49 mins ago










  • $begingroup$
    just curious -- did you write code to do this, or did you do this by hand?
    $endgroup$
    – antkam
    19 mins ago
















$begingroup$
I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
$endgroup$
– Will Jagy
54 mins ago




$begingroup$
I need a better atlas. I'm looking at the Philadelphia area, I cannot tell what happens among Pennsylvania, New Jersey, Delaware, Maryland.
$endgroup$
– Will Jagy
54 mins ago












$begingroup$
@WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
$endgroup$
– Misha Lavrov
53 mins ago




$begingroup$
@WillJagy The reference I actually used to color the US was this picture of the US graph, which solves this problem.
$endgroup$
– Misha Lavrov
53 mins ago












$begingroup$
That's pretty good. A simple standard: at least one drivable road between neighbors
$endgroup$
– Will Jagy
49 mins ago




$begingroup$
That's pretty good. A simple standard: at least one drivable road between neighbors
$endgroup$
– Will Jagy
49 mins ago












$begingroup$
just curious -- did you write code to do this, or did you do this by hand?
$endgroup$
– antkam
19 mins ago




$begingroup$
just curious -- did you write code to do this, or did you do this by hand?
$endgroup$
– antkam
19 mins ago


















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