%I A000644
%S A000644 1,1,3,7,31,138,808,4956,33047,227328
%N A000644 Loops of length 4n on square grid that turn at each step and return to 
               start in original direction.
%D A000644 D. E. Knuth, The Texbook, p. 390.
%D A000644 D. E. Knuth, personal communication.
%Y A000644 Sequence in context: A123488 A121620 A042271 this_sequence A015459 A115083 
               A141385
%Y A000644 Adjacent sequences: A000641 A000642 A000643 this_sequence A000645 A000646 
               A000647
%K A000644 nonn,nice
%O A000644 0,3
%A A000644 N. J. A. Sloane (njas(AT)research.att.com).