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 A073951 #18 May 03 2019 03:04:17
%S A073951 0,1,3,6,21,81,252,729,2187,6642,19845,59049,176904,531441,1594323,
%T A073951 4780782,14344533,43046721,129146724,387420489,1162261467,3486843450,
%U A073951 10460471301,31381059609,94143001680,282429536481,847288609443,2541864234006,7625594296341
%N A073951 Number of strings over Z_3 of length n with trace 1 and subtrace 1.
%C A073951 Same as number of strings over Z_3 of length n with trace 2 and subtrace 1. Same as number of strings over GF(3) of length n with trace 1 and subtrace 1. Same as number of strings over GF(3) of length n with trace 2 and subtrace 1.
%H A073951 F. Ruskey, <a href="http://combos.org/TSstringZ3">Strings over Z_3 with given trace and subtrace</a>
%H A073951 F. Ruskey, <a href="http://combos.org/TSstringF3">Strings over GF(3) with given trace and subtrace</a>
%H A073951 Max Alekseyev, <a href="http://home.gwu.edu/~maxal/gpscripts/">PARI/GP scripts for miscellaneous math problems</a>
%H A073951 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (6,-15,27,-36,27).
%F A073951 a(n; t, s) = a(n-1; t, s) + a(n-1; t+2, s+2t+1) + a(n-1; t+1, s+t+1) where t is the trace and s is the subtrace.
%F A073951 G.f.: q^2(3*q^3+3*q^2-3*q+1)/[(1-3q)(1+3q^2)(1-3q+3q^2)]. - Lawrence Sze, Oct 24 2004
%e A073951 a(2;2,1)=1 since the one ternary string of trace 2, subtrace 1 and length 2 is { 11 }.
%Y A073951 Cf. A073947, A073948, A073949, A073950, A073952.
%K A073951 easy,nonn
%O A073951 1,3
%A A073951 _Frank Ruskey_ and Nate Kube, Aug 15 2002
%E A073951 Terms a(21) onward from _Max Alekseyev_, Apr 09 2013