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%I #6 Jun 25 2018 12:46:14
%S 1,4,19,128,861,6051
%N Number of unrooted self-avoiding walks with n steps that can make turns from the set +-Pi/9, +-3*Pi/9, +-5*Pi/9, +-7*Pi/9.
%C The turning angles are those of the regular nonagon, with one vertex corresponding to the forbidden U-turn. The path may neither intersect nor touch itself anywhere.
%H Contest Organizers, <a href="http://www.recmath.org/contest/Snakes/index.php">Al Zimmermann's Programming Contests - Snakes on a Plane</a>, Fall 2006.
%H Hugo Pfoertner, <a href="http://www.randomwalk.de/sequences/a306179.htm">Illustration of walks of lengths <= 3</a>.
%Y Cf. A266925, A037245, A306175, A151541, A306177, A306178, A306180, A306181, A306182.
%K nonn,walk,more
%O 1,2
%A _Hugo Pfoertner_, Jun 23 2018
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