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 A352388 Number of solutions to Snake Number Problem for snakes with n-periodic instructions in an infinite square grid (see Comments). 2
 0, 0, 0, 0, 5, 21, 127, 618, 2934, 13542, 61803, 276650, 1219508, 5309179, 22868295, 97663066, 414156142, 1746438478 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Given a list of n move instructions (up, right, down, left), the snake starts at the origin and moves according to the instructions, in order. If an instruction tells it to move to a square that has already been visited, the snake skips that instruction. After it has followed (or skipped) the last instruction in the list, it starts again with the first one. a(n) is the number of lists of n instructions that results in the snake getting stuck at some point. Lists of instructions that are equivalent under rotations and reflections are counted only once, so we can for example assume that the first instruction is "up", and that the first "right" comes before the first "left". But how does one know when to interrupt a snake and deem it infinite? - Pontus von Brömssen, May 05 2022 Computer solutions a(5) to a(13) found by Giorgio Vecchi. Computer solution a(14) to a(18) found by Ariel Futoransky. LINKS Table of n, a(n) for n=1..18. Ariel Futoransky, Snake Program, Snake Program to try the snakes, April 2022 (see bottom animation and label for different options). Rodolfo Kurchan, Puzzle Fun, Snake Number Problem, March 2022. EXAMPLE These are the 5 different solutions with 5 instructions: UURDL: 19 17 18 3 4 -- 16 19 2 5 6 15 14 1 8 7 -- 13 10 9 -- -- 12 11 -- -- URDLL: 21 -- 19 20 -- -- -- 17 18 21 2 3 -- 16 15 14 1 4 5 -- 12 13 8 7 6 -- 11 10 9 -- -- URRDL: 24 -- -- 20 21 22 17 18 19 24 23 16 15 2 3 4 13 14 1 6 5 12 11 8 7 -- -- 10 9 -- -- URDLU: 26 -- -- 21 22 -- 16 17 20 23 24 15 18 19 26 25 14 13 2 3 -- -- 12 1 4 5 -- 11 10 7 6 -- -- 9 8 -- URDDL: 30 -- -- 20 21 -- -- -- 18 19 22 23 -- 16 17 2 3 24 -- 15 14 1 4 25 26 12 13 6 5 30 27 11 10 7 -- 29 28 -- 9 8 -- -- -- CROSSREFS Cf. A353259, A353060, A353176, A004147. Sequence in context: A208593 A213009 A316102 * A050910 A220378 A230645 Adjacent sequences: A352385 A352386 A352387 * A352389 A352390 A352391 KEYWORD nonn,more AUTHOR Rodolfo Kurchan, Apr 29 2022 STATUS approved

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Last modified April 17 07:13 EDT 2024. Contains 371756 sequences. (Running on oeis4.)