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A000644 Loops of length 4n on square grid that turn at each step and return to start in original direction. 1
1, 1, 3, 7, 31, 138, 808, 4956, 33047, 227328, 1614793, 11710299, 86495618, 648399937, 4923728056 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Loops cannot overlap and rotations and reflections of a given loop are not counted. - Sean A. Irvine, Oct 18 2015

REFERENCES

D. E. Knuth, The Texbook, p. 390.

D. E. Knuth, personal communication.

LINKS

Table of n, a(n) for n=1..15.

Bert Dobbelaere, Illustration of initial terms

Douglas A. Torrance, Enumeration of planar Tangles, arXiv:1906.01541 [math.CO], 2019.

CROSSREFS

Sequence in context: A123488 A121620 A042271 * A015459 A115083 A141385

Adjacent sequences:  A000641 A000642 A000643 * A000645 A000646 A000647

KEYWORD

nonn,more,nice,walk

AUTHOR

N. J. A. Sloane.

EXTENSIONS

a(11) from Sean A. Irvine, Oct 18 2015

Offset corrected and a(12)-a(15) from Bert Dobbelaere, Jan 11 2020

STATUS

approved

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Last modified July 26 13:40 EDT 2021. Contains 346294 sequences. (Running on oeis4.)