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 A072258 #12 Sep 08 2022 08:45:06
%S A072258 0,9,69,405,2133,10581,50517,234837,1070421,4805973,21321045,93672789,
%T A072258 408245589,1767200085,7605671253,32570168661,138870609237,
%U A072258 589842175317,2496807654741,10536986432853
%N A072258 a(n) = ((6*n+1)*4^n - 1)/3.
%C A072258 Related to Collatz function (for n>0). All divisible by 3.
%H A072258 G. C. Greubel, <a href="/A072258/b072258.txt">Table of n, a(n) for n = 0..1000</a>
%H A072258 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (9,-24,16).
%F A072258 G.f.: 3*x*(3-4*x)/((1-x)*(1-4*x)^2). - _Bruno Berselli_, Dec 16 2011
%F A072258 E.g.f.: ( (1 + 24*x)*exp(4*x) - exp(x) )/3. - _G. C. Greubel_, Jan 14 2020
%p A072258 seq( ((6*n+1)*4^n -1)/3, n=0..40); # _G. C. Greubel_, Jan 14 2020
%t A072258 LinearRecurrence[{9,-24,16}, {0,9,69}, 40] (* _G. C. Greubel_, Jan 14 2020 *)
%o A072258 (PARI) a(n)=((6*n+1)*4^n-1)/3 \\ _Charles R Greathouse IV_, Oct 07 2015
%o A072258 (Magma) [((6*n+1)*4^n -1)/3: n in [0..40]]; // _G. C. Greubel_, Jan 14 2020
%o A072258 (Sage) [((6*n+1)*4^n -1)/3 for n in (0..40)] # _G. C. Greubel_, Jan 14 2020
%o A072258 (GAP) List([0..40], n-> ((6*n+1)*4^n -1)/3); # _G. C. Greubel_, Jan 14 2020
%Y A072258 Cf. A072257, A072259, A072260.
%K A072258 nonn,easy
%O A072258 0,2
%A A072258 N. Rathankar (rathankar(AT)yahoo.com), Jul 08 2002
%E A072258 Edited and extended by _Henry Bottomley_, Aug 06 2002