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 A166925 Digital root of square of n-th triangular number. 1
 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9, 1, 9, 9 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Period is (1,9,9). LINKS Index entries for linear recurrences with constant coefficients, signature (0,0,1). FORMULA a(n)=(1/9)*{43*(n mod 3)+19*[(n+1) mod 3]-5*[(n+2) mod 3]}, with n>=1 [From Paolo P. Lava, Nov 02 2009] G.f.:  x*(1+9*x+9*x^2)/(1-x^3)  - Bruno Berselli, May 05 2011 MATHEMATICA Table[If[(m9 = Mod[(n(n + 1))^2/4, 9]) == 0, 9, m9], {n, 200}] PROG (MAGMA) &cat[[1, 9, 9]: n in [1..29]];  // Bruno Berselli, May 05 2011 CROSSREFS Cf. A000217, A000537, A010888. Sequence in context: A145280 A144667 A118428 * A178164 A216035 A171487 Adjacent sequences:  A166922 A166923 A166924 * A166926 A166927 A166928 KEYWORD base,less,easy,nonn AUTHOR Zak Seidov, Oct 23 2009 STATUS approved

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Last modified July 20 16:34 EDT 2019. Contains 325185 sequences. (Running on oeis4.)