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A166925 Digital root of square of n-th triangular number. 1

%I #26 Dec 12 2023 07:44:33

%S 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,

%T 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,

%U 9,1,9,9,1,9,9,1,9,9,1,9,9,1,9,9,1,9,9

%N Digital root of square of n-th triangular number.

%C Period is (1,9,9).

%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,1).

%F G.f.: x*(1+9*x+9*x^2)/(1-x^3). - _Bruno Berselli_, May 05 2011

%F a(n) = A010888(A000537(n)). - _Michel Marcus_, Mar 17 2023

%t Table[If[(m9 = Mod[(n(n + 1))^2/4, 9]) == 0, 9, m9], {n, 200}]

%o (Magma) &cat[[1,9,9]: n in [1..29]]; // Bruno Berselli, May 05 2011

%Y Cf. A000217, A000537, A010888.

%K base,less,easy,nonn

%O 1,2

%A _Zak Seidov_, Oct 23 2009

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Last modified March 28 08:22 EDT 2024. Contains 371236 sequences. (Running on oeis4.)