In the graph below, each node is coloured either red or yellow, except for the white node in the bottom left, which I've marked with an X.

enter image description here

Can you tell me what the white node marked with X represents?
Should it be coloured red or yellow?

  • $\begingroup$ Maybe this is significant to the puzzle, but the arrow in the upper right is bugging me because it makes this structure not a graph. Can you please clarify? $\endgroup$ – Jeremy Dover Jan 1 at 18:31
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    $\begingroup$ @JeremyDover I've taken your comment into consideration and I've changed the colour of the arrow as I think you make a valid point. $\endgroup$ – hexomino Jan 1 at 18:37
  • $\begingroup$ @bobble I've removed the mathematics tag because I'm not sure that this is more mathematical than what is already covered by the graph-theory tag. However, if there is an issue with that reasoning I don't mind adding it back in. $\endgroup$ – hexomino Jan 1 at 18:40

The white node represents:

GREECE and should be Yellow.

Because this graph represents:

Borders between member states of the European Union. Each node is a country, each line is a land border, and the colours are yellow if the country uses the Euro or red if it does not. The countries can be identified as follows (with one ambiguity where the island nations of Malta and Cyprus are interchangeable):

Node graph resolved
The node marked 'G' is Germany, and the node marked 'F' is France.

As mentioned above, Greece should be yellow because it uses the Euro as its currency. Out of interest, the nations in red use the following currencies: Bulgaria = lev, Croatia = kuna, Czechia = koruna, Denmark = krone, Hungary = forint, Poland = zloty, Romania = leu, Sweden = krona, United Kingdom = pound.

Of further note here is that the UK has a red arrow coming off away from it, since this signifies its recent departure from the European Union. This is particularly pertinent at the start of this new year, since as of the turn of the year the UK has now left the EU's trade arrangements (with a last-minute deal!), effectively severing the final key ties between it and the organisation. (Insert political opinion of your choice here...)

For comparison, here is the real deal:

Map of EU countries, red on the graph circled in red here Source: Maproom, modified

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    $\begingroup$ The lower of the two Netherlands in your diagram should be Luxembourg. $\endgroup$ – WimC Jan 2 at 8:13
  • $\begingroup$ @WimC Oops, good catch - that was entirely unintentional. Fixing now, thanks! :) $\endgroup$ – Stiv Jan 2 at 8:26

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