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 A121621 #13 Jul 16 2019 22:00:49
%S A121621 1,2,-5,-46,-119,122,2035,6554,-239,-86158,-341525,-246046,3455641,
%T A121621 17021162,23161315,-128629846,-815616479,-1590277918,4241902555,
%U A121621 37641223154,95420159401,-107655263398,-1671083125805,-5284814079046,584824319281
%N A121621 Real part of (2 + 3i)^n.
%C A121621 A121622 is the companion sequence generated from (3 + 2i).
%H A121621 Nathaniel Johnston, <a href="/A121621/b121621.txt">Table of n, a(n) for n = 0..500</a>
%H A121621 Beata Bajorska-Harapińska, Barbara Smoleń, Roman Wituła, <a href="https://doi.org/10.1007/s00006-019-0969-9">On Quaternion Equivalents for Quasi-Fibonacci Numbers, Shortly Quaternaccis</a>, Advances in Applied Clifford Algebras (2019) Vol. 29, 54.
%H A121621 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (4,-13)
%F A121621 Re(2 + 3i)^n = a(n) = 4*a(n-1) - 13*a(n-2).
%F A121621 G.f.: ( 1-2*x ) / ( 1 - 4*x + 13*x^2 ). - _R. J. Mathar_, Mar 03 2013
%e A121621 a(5) = 122 since (2 + 3i)^5 = (122 - 597i).
%e A121621 a(5) = 122 = 4*(-119) - 13*(-46) = 4*a(4) - 13*a(3).
%p A121621 A121621:=proc(n)global a:if(n=0)then a[0]:=1:elif(n=1)then a[1]:=2:else a[n]:=4*a[n-1]-13*a[n-2]:fi:return a[n]:end:
%p A121621 seq(A121621(n),n=0..20); # _Nathaniel Johnston_, Apr 15 2011
%t A121621 f[n_] := Re[(2 + 3I)^n]; Table[f[n], {n, 0, 24}] (* _Robert G. Wilson v_ *)
%Y A121621 Cf. A121622.
%K A121621 sign,easy
%O A121621 0,2
%A A121621 _Gary W. Adamson_ and Nick Williams, Aug 10 2006
%E A121621 More terms from _Robert G. Wilson v_, Aug 17 2006