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 A274988 Square spiral reflection. 0
 1, 3, 5, 7, 9, 9, 9, 9, 11, 13, 13, 15, 17, 19, 21, 23, 25, 27, 27, 27, 27, 27, 27, 27, 29, 31, 33, 33, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 53, 53, 53, 53, 53, 53, 53, 53, 53, 55, 57, 59, 61 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Draw a square spiral growing by a unit line segment on each step, and at the same time, draw another square spiral in the opposite direction (again using a unit line segment) that turns after the first step (see example). a(n) is the number of unit line segments remaining after the n-th step. Note that overlapping line segments are counted once. LINKS EXAMPLE __     ____      ____      ____     ____     ____    __      __|    __|      __|    |  __|    |  __|   |  __|   |  __|              |    __|    ____|    |____|    |____|   |____|   |____|    n=1     n=2    n=3     n=4       n=5      n=6      n=7      n=8 --                                                          __      __ __ ________     ____ __      ____ __      ____ __|     ____ __|   ____ __| |  __|      |  __|  |    |  __|  |    |  __|  |    |  __|  |  |  __|  | |____|__    |____|__|    |____|__|    |____|__|    |____|__|  |____|__|                                               |          __|     __ __|    n=9        n=10         n=11         n=12          n=13       n=14 CROSSREFS Sequence in context: A139081 A196189 A206543 * A265509 A265527 A217250 Adjacent sequences:  A274985 A274986 A274987 * A274989 A274990 A274991 KEYWORD nonn AUTHOR Wesley Ivan Hurt, Dec 25 2016 STATUS approved

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Last modified September 19 18:29 EDT 2020. Contains 337181 sequences. (Running on oeis4.)