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 A073947 #29 Nov 29 2024 12:49:56
%S A073947 1,1,3,9,21,63,225,729,2187,6561,19845,59535,177633,531441,1594323,
%T A073947 4782969,14344533,43033599,129127041,387420489,1162261467,3486784401,
%U A073947 10460471301,31381413903,94143533121,282429536481,847288609443,2541865828329,7625594296341
%N A073947 Number of strings over Z_3 of length n with trace 0 and subtrace 0.
%C A073947 Same as number of strings over GF(3) of length n with trace 0 and subtrace 0.
%H A073947 Harvey P. Dale, <a href="/A073947/b073947.txt">Table of n, a(n) for n = 1..1000</a>
%H A073947 Max Alekseyev, <a href="http://home.gwu.edu/~maxal/gpscripts/">PARI/GP scripts for miscellaneous math problems</a>
%H A073947 F. Ruskey, <a href="http://combos.org/TSstringZ3">Strings over Z_3 with given trace and subtrace</a>
%H A073947 F. Ruskey, <a href="http://combos.org/TSstringF3">Strings over GF(3) with given trace and subtrace</a>
%H A073947 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (6,-15,27,-36,27).
%F A073947 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 A073947 G.f.: q*(21*q^4-21*q^3+12*q^2-5*q+1)/[(1-3q)(1+3q^2)(1-3q+3q^2)]. - Lawrence Sze, Oct 24 2004
%e A073947 a(3;0,0)=3 since the three ternary strings of trace 0, subtrace 0 and length 3 are { 000, 111, 222 }.
%t A073947 LinearRecurrence[{6,-15,27,-36,27},{1,1,3,9,21},40] (* _Harvey P. Dale_, Nov 29 2024 *)
%Y A073947 Cf. A073948, A073949, A073950, A073951, A073952.
%K A073947 easy,nonn
%O A073947 1,3
%A A073947 _Frank Ruskey_ and Nate Kube, Aug 15 2002
%E A073947 Terms a(21) onward from _Max Alekseyev_, Apr 09 2013