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Do you have a proof that 13 is optimal? I can't think of one.
Work in your variable setup with a0 through a6. You were wondering how to handle countries that are more than sextuply-landlocked; I think the easiest way is to dump them into a6 regardless of how excessively landlocked they are. We want to prove that if a0,...,a6 >= 1 and a0+...+a6 <= 12, then the sum of ai(a(i-1)+ai+a(i+1)-1) is smaller than 6*(a0+...+a6). A simple way to do that is to enumerate all possible tuples, which took a few minutes of programming for me and a few milliseconds of runtime on my computer.
You can also work out a proof witho... (read more)