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%I #9 Jan 10 2019 06:04:39
%S 0,0,0,0,2,19,150,1043,6843,43192,266529,1619983,9746883,58220994,
%T 345919915
%N Number of self-trapped uncrossed king's paths on an infinite board after n steps, reduced for symmetry.
%e a(5) = 2: There are 2 walks where the king is blocked after 5 steps, because for the diagonal moves it would have to cross its previous path.
%e .
%e o 2 o o 3 o
%e / \ / \
%e / \ / \
%e / \ / \
%e 3 5 1 4 - - - 5 2
%e | / / /
%e | / / /
%e | / / /
%e 4 S o S - - - 1 o
%Y Cf. A003192, A077482, A272773, A323140.
%K nonn,more,hard
%O 1,5
%A _Hugo Pfoertner_, Jan 05 2019
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