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 A076506 #24 Oct 05 2019 04:54:59
%S A076506 1,3,12,24,72,288,576,1728,6912,13824,41472,165888,331776,995328,
%T A076506 3981312,7962624,23887872,95551488,191102976,573308928,2293235712,
%U A076506 4586471424,13759414272,55037657088,110075314176,330225942528
%N A076506 Expansion of x*(1+3*x+12*x^2)/(1-24*x^3).
%C A076506 Previous name was: 3 people at a party are saying Hello to each other. Person 1 says Hello. Person 2 counts the times Hello has been said and says Hello twice that number of times. Person 3 says Hello 3 times the sum of Hello's and then it is Person 1's turn again. This is how the sum of Hello's.
%H A076506 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,24).
%F A076506 G.f.: x*(1+3*x+12*x^2)/(1-24*x^3). [_Colin Barker_, Jun 07 2012]
%t A076506 a[n_] := Product[Mod[k, 3]+2, {k, 1, n-1}]; Array[a, 26] (* _Jean-François Alcover_, Oct 05 2016, adapted from PARI *)
%o A076506 (PARI) a(n)=prod(k=1,n-1,k%3+2)
%Y A076506 Cf. A076505.
%K A076506 nonn,easy
%O A076506 1,2
%A A076506 _Jon Perry_, Oct 15 2002