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 A265409 a(n) = index to the nearest inner neighbor in Ulam-style square-spirals using zero-based indexing. 6
 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 4, 4, 4, 5, 6, 6, 6, 7, 8, 9, 9, 9, 10, 11, 12, 12, 12, 13, 14, 15, 16, 16, 16, 17, 18, 19, 20, 20, 20, 21, 22, 23, 24, 25, 25, 25, 26, 27, 28, 29, 30, 30, 30, 31, 32, 33, 34, 35, 36, 36, 36, 37, 38, 39, 40, 41, 42, 42, 42, 43, 44, 45, 46, 47, 48, 49, 49, 49, 50 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,11 COMMENTS Each n occurs A265411(n+1) times. Useful when defining recurrences like A078510 and A265408. LINKS Antti Karttunen, Table of n, a(n) for n = 1..10000 FORMULA If n <= 7, a(n) = 0 for n >= 8: if either A240025(n) or A240025(n-1) is not zero [when n or n-1 is in A002620], then a(n) = a(n-1), otherwise, a(n) = 1 + a(n-1). If n <= 7, a(n) = 0, for n >= 8, a(n) = a(n-1) + (1-A240025(n))*(1-A240025(n-1)). [The same formula in a more compact form.] a(n) = A265410(n+1) - 1. Other identities. For all n >= 0: a(n) = n - A265359(n). EXAMPLE We arrange natural numbers as a counterclockwise spiral into the square grid in the following manner (here A stands for 10, B for 11). The first square corresponds with n (where the initial term 0 is at the center), and the second square with the value of a(n). This sequence doesn't specify a(0), thus it is shown as an asterisk (*):                     44322             432B    40002B             501A    50*01A             6789    600119                     667899 - For each n > 0, we look for the nearest horizontally or vertically adjacent neighbor of n towards the center that is not n-1, which will then be value of a(n) [e.g., it is 0 for 3, 5 and 7, while it is 1 for 8, 9 and A (10) and 2 for B (11)] unless n is in the corner (one of the terms of A002620), in which case the value is the nearest diagonally adjacent neighbor towards the center, e.g. 0 for 2, 4 and 6, while it is 1 for 9). See also the illustration at A078510. PROG (Scheme, with memoization macro definec) (definec (A265409 n) (cond ((<= n 7) 0) ((or (= 1 (A240025 n)) (= 1 (A240025 (- n 1)))) (A265409 (- n 1))) (else (+ 1 (A265409 (- n 1)))))) CROSSREFS One less than A265410(n+1). Cf. A002620, A240025, A265359. Cf. also A033951, A063826, A078510, A265408. Sequence in context: A194169 A194165 A004524 * A126257 A025773 A308303 Adjacent sequences:  A265406 A265407 A265408 * A265410 A265411 A265412 KEYWORD nonn AUTHOR Antti Karttunen, Dec 13 2015 STATUS approved

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Last modified July 4 02:21 EDT 2020. Contains 335436 sequences. (Running on oeis4.)