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 A332150 #12 Jun 30 2025 13:48:43
%S A332150 0,505,55055,5550555,555505555,55555055555,5555550555555,
%T A332150 555555505555555,55555555055555555,5555555550555555555,
%U A332150 555555555505555555555,55555555555055555555555,5555555555550555555555555,555555555555505555555555555,55555555555555055555555555555,5555555555555550555555555555555
%N A332150 a(n) = 5*(10^(2n+1)-1)/9 - 5*10^n.
%H A332150 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332150 a(n) = 5*A138148(n) = A002279(2n+1) - 5*10^n.
%F A332150 G.f.: 5*x*(101 - 200*x)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332150 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332150 A332150 := n -> 5*((10^(2*n+1)-1)/9-10^n);
%t A332150 Array[5 ((10^(2 # + 1)-1)/9 - 10^#) &, 15, 0]
%t A332150 Table[With[{c=PadRight[{},n,5]},FromDigits[Join[c,{0},c]]],{n,0,15}] (* or *) LinearRecurrence[{111,-1110,1000},{0,505,55055},20] (* _Harvey P. Dale_, Jun 30 2025 *)
%o A332150 (PARI) apply( {A332150(n)=(10^(n*2+1)\9-10^n)*5}, [0..15])
%o A332150 (Python) def A332150(n): return (10**(n*2+1)//9-10**n)*5
%Y A332150 Cf. A002275 (repunits R_n = (10^n-1)/9), A002279 (5*R_n), A011557 (10^n).
%Y A332150 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332150 Cf. A332120 .. A332190 (variants with different repeated digit 2, ..., 9).
%Y A332150 Cf. A332151 .. A332159 (variants with different middle digit 1, ..., 9).
%K A332150 nonn,base,easy
%O A332150 0,2
%A A332150 _M. F. Hasler_, Feb 09 2020