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 A121338 #14 Aug 16 2015 12:03:57
%S A121338 70,511258935,3732600255368600,27250975409595074561065,
%T A121338 198953975772318806945317308330,1452523584226469439408576900215922395,
%U A121338 10604587088767577582197244731443261336155260,77421990626847055423676582260371016672624778798925
%N A121338 Pentagonal numbers P(k) that are one-third of other pentagonal numbers: P(k) such that 3*P(k)=P(m) for some m>k.
%C A121338 The k values are (A001835(6n-2)+1)/6, the m values are (A001834(6n-3)+1)/6.
%H A121338 Colin Barker, <a href="/A121338/b121338.txt">Table of n, a(n) for n = 1..146</a>
%H A121338 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (7300803,-7300803,1).
%F A121338 a(n) = ((A001835(6n-2))^2-1)/24.
%F A121338 a(n) = 7300803*a(n-1)-7300803*a(n-2)+a(n-3). - _Colin Barker_, Jun 20 2015
%F A121338 G.f.: -5*x*(x^2+40545*x+14) / ((x-1)*(x^2-7300802*x+1)). - _Colin Barker_, Jun 20 2015
%e A121338 a(1) = ((A001835(4))^2-1)/24 = (41^2-1)/24 = 70; this number and 3*70=210 are pentagonal numbers (in A000326).
%t A121338 CoefficientList[Series[5 (x^2 + 40545 x + 14)/((1 - x) (x^2 - 7300802 x + 1)), {x, 0, 20}], x] (* _Vincenzo Librandi_, Jun 21 2015 *)
%o A121338 (PARI) Vec(-5*x*(x^2+40545*x+14)/((x-1)*(x^2-7300802*x+1)) + O(x^20)) \\ _Colin Barker_, Jun 20 2015
%Y A121338 Cf. A001834, A001835.
%K A121338 nonn,easy
%O A121338 1,1
%A A121338 _Franz Vrabec_, Aug 28 2006
%E A121338 Added more terms, _Colin Barker_, Jun 20 2015