%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