%I #26 Jun 17 2022 03:18:37
%S 1,1,3,7,31,138,808,4956,33047,227328,1614793,11710299,86495618,
%T 648399937,4923728056
%N Number of loops of length 4n on square grid that turn at each step and return to start in original direction.
%C Loops cannot overlap and rotations and reflections of a given loop are not counted. - _Sean A. Irvine_, Oct 18 2015
%D D. E. Knuth, The Texbook, p. 390.
%D D. E. Knuth, personal communication.
%H Bert Dobbelaere, <a href="/A000644/a000644.pdf">Illustration of initial terms</a>
%H Douglas A. Torrance, <a href="https://arxiv.org/abs/1906.01541">Enumeration of planar Tangles</a>, arXiv:1906.01541 [math.CO], 2019.
%K nonn,more,nice,walk
%O 1,3
%A _N. J. A. Sloane_
%E a(11) from _Sean A. Irvine_, Oct 18 2015
%E Offset corrected and a(12)-a(15) from _Bert Dobbelaere_, Jan 11 2020