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 A002276 #54 Jul 20 2025 08:56:26
%S A002276 0,2,22,222,2222,22222,222222,2222222,22222222,222222222,2222222222,
%T A002276 22222222222,222222222222,2222222222222,22222222222222,
%U A002276 222222222222222,2222222222222222,22222222222222222,222222222222222222,2222222222222222222,22222222222222222222,222222222222222222222
%N A002276 a(n) = 2*(10^n - 1)/9.
%C A002276 a(n) is also the total number of holes in a variation of a box fractal as in illustration. - _Kival Ngaokrajang_, May 23 2014 [As observed by _Hans Havermann_, this seems to be incorrect: e.g., for n = 2 the illustration shows 28 small holes plus two larger holes. - _M. F. Hasler_, Oct 05 2020]
%H A002276 Ivan Panchenko, <a href="/A002276/b002276.txt">Table of n, a(n) for n = 0..200</a>
%H A002276 Kival Ngaokrajang, <a href="/A002276/a002276.pdf">Illustration for n = 1..4</a>.
%H A002276 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A002276 a(n) = A178630(n)/A002283(n). - _Reinhard Zumkeller_, May 31 2010
%F A002276 From _Vincenzo Librandi_, Jul 22 2010: (Start)
%F A002276 a(n) = a(n-1) + 2*10^(n-1) with a(0) = 0.
%F A002276 a(n) = 11*a(n-1) - 10*a(n-2) with a(0) = 0, a(1) = 2. (End)
%F A002276 G.f.: 2*x/((1 - x)*(1 - 10*x)). - _Ilya Gutkovskiy_, Feb 24 2017
%F A002276 E.g.f.: 2*exp(x)*(exp(9*x) - 1)/9. - _Stefano Spezia_, Sep 13 2023
%F A002276 From _Elmo R. Oliveira_, Jul 19 2025: (Start)
%F A002276 a(n) = 2*A002275(n).
%F A002276 a(n) = A010785(A017185(n-1)) for n >= 1. (End)
%t A002276 LinearRecurrence[{11, -10}, {0, 2}, 50] (* _Jinyuan Wang_, Feb 27 2020 *)
%o A002276 (Maxima) A002276(n):=2*(10^n - 1)/9$
%o A002276 makelist(A002276(n),n,0,20); /* _Martin Ettl_, Nov 12 2012 */
%o A002276 (PARI) a(n)=10^n\9*2 \\ _M. F. Hasler_, Mar 27 2015
%Y A002276 Cf. A002275, A002277, A002278, A002279, A002280, A002281, A002282, A002283, A178630, A178634.
%Y A002276 Cf. A010785, A017185.
%K A002276 nonn,easy
%O A002276 0,2
%A A002276 _N. J. A. Sloane_