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 A334044 a(1) = 0, and for any n > 1, a(n) is the number of points of the set { (k, a(k)), k = 1..n-2 } that are not visible from the point (n-1, a(n-1)). 2
 0, 0, 0, 1, 0, 2, 1, 0, 4, 1, 1, 2, 2, 2, 3, 2, 3, 5, 2, 6, 4, 3, 3, 4, 4, 7, 1, 5, 5, 2, 6, 5, 4, 6, 6, 9, 0, 6, 9, 3, 4, 7, 5, 8, 5, 6, 6, 10, 8, 10, 7, 7, 7, 5, 6, 11, 6, 11, 8, 6, 14, 8, 8, 10, 11, 9, 8, 15, 8, 12, 8, 12, 11, 6, 14, 8, 12, 14, 10, 13, 8 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS For any i and k such that i < k: the point (i, a(i)) is visible from the point (k, a(k)) if there are no j such that i < j < k and the three points (i, a(i)), (j, a(j)), (k, a(k)) are aligned. LINKS Rémy Sigrist, Table of n, a(n) for n = 1..10000 EXAMPLE For n = 6: - we consider the following points: . . . X . (4,1) X X X . X (1,0) (2,0) (3,0) (5,0) - the points (1,0) and (2,0) are not visible from (5,0) (as they are hidden by (3,0)), - whereas the points (3,0) and (4,1) are visible from (5,0) - hence a(6) = 2. PROG (PARI) g(z) = z/gcd(real(z), imag(z)) for (n=1, #a=vector(81), print1 (a[n] = max(0, n-2) - #Set(apply(k -> g((k+a[k]*I)-(n-1+a[n-1]*I)), [1..n-2]))", ")) CROSSREFS See A334043 for a similar sequence. Sequence in context: A139360 A326759 A140882 * A143724 A143425 A323376 Adjacent sequences: A334041 A334042 A334043 * A334045 A334046 A334047 KEYWORD nonn AUTHOR Rémy Sigrist, Apr 13 2020 STATUS approved

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Last modified September 22 21:28 EDT 2023. Contains 365531 sequences. (Running on oeis4.)