This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A166925 #30 Sep 03 2025 09:15:05
%S A166925 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 A166925 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 A166925 9,1,9,9,1,9,9,1,9,9,1,9,9,1,9,9,1,9,9
%N A166925 Digital root of square of n-th triangular number.
%C A166925 Period is (1,9,9).
%H A166925 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,1).
%F A166925 G.f.: x*(1+9*x+9*x^2)/(1-x^3). - _Bruno Berselli_, May 05 2011
%F A166925 a(n) = A010888(A000537(n)). - _Michel Marcus_, Mar 17 2023
%t A166925 Table[If[(m9 = Mod[(n(n + 1))^2/4, 9]) == 0, 9, m9], {n, 200}]
%o A166925 (Magma) &cat[[1,9,9]: n in [1..29]]; // _Bruno Berselli_, May 05 2011
%Y A166925 Cf. A000217, A000537, A010888.
%K A166925 base,less,easy,nonn,changed
%O A166925 1,2
%A A166925 _Zak Seidov_, Oct 23 2009