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 A348009 Number of n-step self-avoiding walks on one quadrant of a 2D square lattice where the walk cannot step to the smaller square ring of numbers than the ring it is currently on. 1
 1, 2, 4, 10, 22, 52, 118, 282, 646, 1544, 3576, 8546, 19924, 47612, 111536, 266488, 626520, 1496670, 3528470, 8427952, 19913078, 47559756, 112572916, 268857568, 637327742, 1522153378, 3612811784, 8629110414, 20503211908, 48975965026, 116478744692 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS This is a variation of A347990. The same walk rules apply except that the walk is confined to one quadrant of the 2D square lattice. See A347990 for further details. LINKS EXAMPLE a(0..3) are the same as the standard SAW on one quadrant of a square lattice, see A038373, as the walk cannot step to a smaller ring in the first three steps. a(4) = 22. If we restrict the first one or more steps to the right followed by an upward step then there is one walk which steps to a smaller ring and is thus forbidden. That is the walk (0,0) -> (1,0) -> (2,0) -> (2,1) -> (1,1). As this can be walked in two different ways in one quadrant the number of 4-step walks becomes A038373(4) - 2 = 24 - 2 = 22. CROSSREFS Cf. A347990 (four quadrants), A348008 (two quadrants), A038373, A001411, A337353. Sequence in context: A078040 A240041 A164990 * A121285 A221536 A290277 Adjacent sequences:  A348006 A348007 A348008 * A348010 A348011 A348012 KEYWORD nonn,walk AUTHOR Scott R. Shannon, Sep 24 2021 STATUS approved

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Last modified August 13 17:37 EDT 2022. Contains 356107 sequences. (Running on oeis4.)