OFFSET
0,2
COMMENTS
EXAMPLE
a(0..3) are the same as the standard SAW on the upper two quadrants of a square lattice, see A116903, as the walk cannot step to a smaller ring in the first three steps.
a(4) = 45. 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 four different ways in the upper two quadrants the number of 4-step walks becomes A116903(4) - 4 = 49 - 4 = 45.
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
Scott R. Shannon, Sep 24 2021
STATUS
approved