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 A087509 Number of k such that (k*n) == 2 (mod 3) for 0 <= k <= n. 5
 0, 0, 1, 0, 1, 2, 0, 2, 3, 0, 3, 4, 0, 4, 5, 0, 5, 6, 0, 6, 7, 0, 7, 8, 0, 8, 9, 0, 9, 10, 0, 10, 11, 0, 11, 12, 0, 12, 13, 0, 13, 14, 0, 14, 15, 0, 15, 16, 0, 16, 17, 0, 17, 18, 0, 18, 19, 0, 19, 20, 0, 20, 21, 0, 21, 22, 0, 22, 23, 0, 23, 24, 0, 24, 25, 0, 25, 26, 0, 26, 27, 0, 27, 28, 0, 28 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 LINKS Table of n, a(n) for n=0..85. Index entries for linear recurrences with constant coefficients, signature (0,0,2,0,0,-1). FORMULA a(n) = Sum_{k=0..n} [(k*n) == 2 (mod 3)]; a(n) = n - 2*(floor(n/3) + 1)*(1 - cos(2*Pi*n/3))/3 - floor(n/3)*(5 + 4*cos(2*Pi*n/3))/3. a(n) = n - A087507(n) - A087508(n). G.f.: x^2*(x^2+1) / ((x-1)^2*(x^2+x+1)^2). - Colin Barker, Mar 31 2013 a(n) = 2*sin(n*Pi/3)*(sqrt(3)*cos(n*Pi) + 2*n*sin(n*Pi/3))/9. - Wesley Ivan Hurt, Sep 24 2017 EXAMPLE a(8) = #{1,4,7} = 3. MATHEMATICA {#-1, 1+#, 0}[[Mod[#, 3, 1]]]/3&/@Range[0, 99] (* Federico Provvedi, Jun 15 2021 *) LinearRecurrence[{0, 0, 2, 0, 0, -1}, {0, 0, 1, 0, 1, 2}, 100] (* Harvey P. Dale, May 04 2023 *) PROG (PARI) a(n) = sum(k=0, n, (k*n % 3)==2); \\ Michel Marcus, Sep 25 2017 CROSSREFS Cf. A087507, A087508. Sequence in context: A281260 A182406 A160706 * A274097 A265400 A181871 Adjacent sequences: A087506 A087507 A087508 * A087510 A087511 A087512 KEYWORD easy,nonn AUTHOR Paul Barry, Sep 11 2003 STATUS approved

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Last modified May 17 23:39 EDT 2024. Contains 372608 sequences. (Running on oeis4.)