I think there are tighter groups to be found than some of those listed above... for instance, Jerusalem-Amman-Beirut-Damascus or Tirana-Prstina-Skopje-Podgorica, right ?
For the tightest groups look at http://www.jetpunk.com/user-quizzes/91108/20-closest-sets-of-3-capital-cities and http://www.jetpunk.com/user-quizzes/91108/20-closest-sets-of-capital-cities
This quiz isn't about tighter "groups" though, it's about "special" capital cities placed in a way that they are the closest capital city there is for at least 3 other capital cities.
For instance, reguarding your first example, the four capitals you mentioned are "paired up", so Amman and Jerusalem are reciprocally closest and so are Damascus and Beirut; this means that none of the 4 is a "special" one
Thanks for the clarification, got it now ! Thanks for the links as well - I hadn't seen those other quizzes before. The 'specialty' meant here is abstract in such a way that it is actually difficult to find a good, short title for it. But it's a great quiz and a hard one, too !
This quiz isn't about tighter "groups" though, it's about "special" capital cities placed in a way that they are the closest capital city there is for at least 3 other capital cities.
For instance, reguarding your first example, the four capitals you mentioned are "paired up", so Amman and Jerusalem are reciprocally closest and so are Damascus and Beirut; this means that none of the 4 is a "special" one
Hope that made sense!