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 A205650 Period 12: repeat (1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9). 0
 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9, 1, 6 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The members of this sequence are also the digital roots of the indices of those nonzero square numbers that are also triangular. The coefficients of x^n in the numerator of the generating function form the periodic cycle of the sequence. LINKS Table of n, a(n) for n=1..86. Index entries for linear recurrences with constant coefficients, signature (0,0,0,0,0,0,0,0,0,0,0,1). FORMULA G.f. x*(1 + 6*x + 8*x^2 + 6*x^3 + x^4 + 9*x^5 + 8*x^6 + 3*x^7 + x^8 + 3*x^9 + 8*x^10 + 9*x^11) / (1 - x^12). a(n) = a(n-12). a(n) = 63-a(n-1)-a(n-2)-a(n-3)-a(n-4)-a(n-5)-a(n-6)-a(n-7)-a(n-8)-a(n-9)-a(n-10)-a(n-11). a(n) = 1/4*(21+(-1)^n*(3+14*sin(n*pi/2))+6*((-1)^n+1)*cos(2*n*pi/3)-2*sqrt(3)*(-1)^n*(sin(5*n*pi/6)-sin(n*pi/6))). For n>0, a(n) = A010888(A001109(n)) = A010888(sqrt(A001110(n))). EXAMPLE The fourth nonzero square number that is also a triangular number is 204^2. As 204 has digital root 2+4=6, then a(4)=6. MATHEMATICA LinearRecurrence[{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}, {1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9}, 86] PadRight[{}, 90, {1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8, 9}] (* Harvey P. Dale, Apr 10 2013 *) PROG (PARI) a(n)=[9, 1, 6, 8, 6, 1, 9, 8, 3, 1, 3, 8][n%12+1] \\ Charles R Greathouse IV, Jul 17 2016 CROSSREFS Cf. A010888, A001109, A001110. Sequence in context: A266092 A143627 A105798 * A153755 A331374 A340725 Adjacent sequences: A205647 A205648 A205649 * A205651 A205652 A205653 KEYWORD nonn,easy AUTHOR Ant King, Jan 30 2012 STATUS approved

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Last modified February 23 21:40 EST 2024. Contains 370288 sequences. (Running on oeis4.)