Neutrondecay asks which pairs of countries border exactly the same other countries? When the set is non-empty? When the set is more than one other country? If you allow pairs that also border each other?
Process
After thinking about it a bit and not getting anywhere, I took http://en.wikipedia.org/wiki/List_of_countries_and_territories_by_land_borders, tidied it up a lot with some hacky regexes, and sorted by the "borders" column.
N=0: Island states
Half the list borders no country at all. I didn't bother to list all combination of pairs :)
Enclaves
There's a few examples of two countries enclaved in the same large country, or sandwiched between the same country and the sea.
East Timor & Papua New Guinea border: Indonesia
San Marino & Vatican City border: Italy
Hong Kong & Macau border: PRC
Portugal & Gibralter border: Spain
Non-trivial examples
Bhutan & Nepal border: PRC, India
This was the only non-trivial example I found, but I scanned by eye and may have missed some.
ETA: UAE and Yemen border only {Oman, Saudi Arabia}. (Moved from below, put it in the wrong place to start with.)
Border only each other
Take http://en.wikipedia.org/wiki/List_of_countries_that_border_only_one_other_country and search for "island".
There were some more questionable cases not on this list, but it gives:
Haiti & Dominican Republic
UK & Republic of Ireland
Countries which border each other and the same set of other countries
I couldn't find any on the list, but I may have been missing some.
ETA: James Cranch points out:
Finland and Norway both border only {Sweden, Russia}
Conclusions
I think this is about what you'd expect -- most countries border countries of about the same order of magnitude, which means one of them normally borders _some_ other country, unless you have several tiny countries, or sandwiched together on an island.
It would be possible for two countries to border the same three other countries -- eg. three countries sideways across a peninsula all touching another country at the top and one on the mainland. But I couldn't think of any.
Can anyone do better?
Process
After thinking about it a bit and not getting anywhere, I took http://en.wikipedia.org/wiki/List_of_countries_and_territories_by_land_borders, tidied it up a lot with some hacky regexes, and sorted by the "borders" column.
N=0: Island states
Half the list borders no country at all. I didn't bother to list all combination of pairs :)
Enclaves
There's a few examples of two countries enclaved in the same large country, or sandwiched between the same country and the sea.
East Timor & Papua New Guinea border: Indonesia
San Marino & Vatican City border: Italy
Hong Kong & Macau border: PRC
Portugal & Gibralter border: Spain
Non-trivial examples
Bhutan & Nepal border: PRC, India
This was the only non-trivial example I found, but I scanned by eye and may have missed some.
ETA: UAE and Yemen border only {Oman, Saudi Arabia}. (Moved from below, put it in the wrong place to start with.)
Border only each other
Take http://en.wikipedia.org/wiki/List_of_countries_that_border_only_one_other_country and search for "island".
There were some more questionable cases not on this list, but it gives:
Haiti & Dominican Republic
UK & Republic of Ireland
Countries which border each other and the same set of other countries
I couldn't find any on the list, but I may have been missing some.
ETA: James Cranch points out:
Finland and Norway both border only {Sweden, Russia}
Conclusions
I think this is about what you'd expect -- most countries border countries of about the same order of magnitude, which means one of them normally borders _some_ other country, unless you have several tiny countries, or sandwiched together on an island.
It would be possible for two countries to border the same three other countries -- eg. three countries sideways across a peninsula all touching another country at the top and one on the mainland. But I couldn't think of any.
Can anyone do better?