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 A180599 #51 Sep 22 2023 12:16:15
%S A180599 0,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,
%T A180599 9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,
%U A180599 9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9
%N A180599 Zero followed by infinitely many 9's.
%C A180599 Another interpretation: A real number with an infinitesimally small difference from the integer 1 which is used to test the precision of calculating devices. - _John W. Nicholson_, Feb 01 2012
%C A180599 a(n) is also the number of n-digit positive repdigit numbers (A010785). - _Stefano Spezia_, Aug 15 2020
%H A180599 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (1).
%F A180599 a(0) = 0, a(n) = 9 for n > 0.
%F A180599 a(n) = 9 * A057427(n).
%F A180599 a(n) = A010888(9*n), where A010888 is the digital root.
%F A180599 From _Robert Israel_, Dec 16 2014: (Start)
%F A180599 G.f.: 9*x/(1 - x).
%F A180599 E.g.f.: 9*(exp(x) - 1). (End)
%e A180599 Viewed as a real number: For a TI-89, entering 1.-10^-12 yields .999999999999; however, 1.-10^-13 yields 1. - _John W. Nicholson_, Feb 01 2012
%t A180599 a[n_] := Mod[9 n - 1, 9] + 1; a[0] = 0; Array[a, 105, 0] (* _Robert G. Wilson v_, Sep 20 2010 *)
%t A180599 PadRight[{0},120,9] (* _Harvey P. Dale_, May 03 2019 *)
%o A180599 (PARI) a(n)=if(n,9,0) \\ _Charles R Greathouse IV_, Jan 04 2013
%Y A180599 Cf. A007953, A010785, A010888, A057427, A180592, A180594, A180595, A180596, A180597, A180598.
%K A180599 easy,nonn
%O A180599 0,2
%A A180599 _Odimar Fabeny_, Sep 10 2010
%E A180599 More terms from _Robert G. Wilson v_, Sep 20 2010
%E A180599 Definition changed by _N. J. A. Sloane_, Feb 04 2012