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 A338712 #37 Aug 31 2025 02:48:14
%S A338712 1,3,7,9,11,33,77,99,111,333,777,999,1111,3333,7777,9999,11111,33333,
%T A338712 77777,99999,111111,333333,777777,999999,1111111,3333333,7777777,
%U A338712 9999999,11111111,33333333,77777777,99999999,111111111,333333333,777777777,999999999,1111111111,3333333333,7777777777,9999999999
%N A338712 Numbers with all digits equal and from the set {1, 3, 7, 9}.
%C A338712 Candidates for prefixes and suffixes in A090287.
%H A338712 Robert Price, <a href="/A338712/b338712.txt">Table of n, a(n) for n = 1..1000</a>
%H A338712 <a href="/index/Rec#order_08">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,11,0,0,0,-10).
%F A338712 From _Colin Barker_, Nov 09 2020: (Start)
%F A338712 G.f.: x*(1 + 3*x + 7*x^2 + 9*x^3) / ((1 - x)*(1 + x)*(1 + x^2)*(1 - 10*x^4)).
%F A338712 a(n) = 11*a(n-4) - 10*a(n-8) for n>8. (End)
%F A338712 Sum_{n>=1} 1/a(n) = (100/63) * A065444. - _Amiram Eldar_, Aug 31 2025
%p A338712 a:= n-> [1, 3, 7, 9][1+irem(n-1, 4)]*(10^iquo(n+3, 4)-1)/9:
%p A338712 seq(a(n), n=1..50); # _Alois P. Heinz_, Nov 09 2020
%t A338712 A338712={}; Do[AppendTo[A338712, FromDigits[ConstantArray[#,i]] & /@{ 1,3,7,9}], {i,10}]; A338712//Flatten (* _Robert Price_, Sep 21 2023 *)
%Y A338712 Cf. A065444, A090287, A338366.
%K A338712 nonn,base,easy,changed
%O A338712 1,2
%A A338712 _N. J. A. Sloane_, Nov 08 2020, following a suggestion from _Hugo Pfoertner_