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A037245 Number of unrooted self-avoiding walks of n steps on square lattice. 16
1, 2, 4, 9, 22, 56, 147, 388, 1047, 2806, 7600, 20437, 55313, 148752, 401629, 1078746, 2905751, 7793632, 20949045, 56112530, 150561752, 402802376, 1079193821, 2884195424, 7717665979, 20607171273, 55082560423, 146961482787, 392462843329, 1046373230168, 2792115083878 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Or, number of 2-sided polyedges with n cells. - Ed Pegg Jr, May 13 2009

A walk and its reflection (i.e., exchange start and end of walk, what Hayes calls a "retroreflection") are considered one and the same here. - Joerg Arndt, Jan 26 2018

With A001411 as main input and counting the symmetrical shapes separately, higher terms can be computed efficiently (see formula). - Bert Dobbelaere, Jan 07 2019

LINKS

Bert Dobbelaere, Table of n, a(n) for n = 1..60

Joerg Arndt, The a(6) = 56 walks of length 6, 2018 (pdf, 2 pages).

Brian Hayes, How to avoid yourself, American Scientist 86 (1998) 314-319.

Ed Pegg, Jr., Illustrations of polyforms

Eric Weisstein's World of Mathematics, Polyedge

FORMULA

a(n) = (A001411(n) + A323188(n) + A323189(n) + 4) / 16. - Bert Dobbelaere, Jan 07 2019

CROSSREFS

Asymptotically approaches (1/16) * A001411.

Cf. A266549 (closed self-avoiding walks).

Cf. A323188, A323189 (program).

Sequence in context: A091561 A025265 A152225 * A244886 A143017 A307575

Adjacent sequences:  A037242 A037243 A037244 * A037246 A037247 A037248

KEYWORD

nonn,walk,hard,nice

AUTHOR

Brian Hayes

EXTENSIONS

a(25)-a(27) from Luca Petrone, Dec 20 2015

More terms using formula by Bert Dobbelaere, Jan 07 2019

STATUS

approved

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Last modified May 31 15:41 EDT 2020. Contains 334748 sequences. (Running on oeis4.)