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 A332157 #6 Feb 11 2020 08:17:29
%S A332157 7,575,55755,5557555,555575555,55555755555,5555557555555,
%T A332157 555555575555555,55555555755555555,5555555557555555555,
%U A332157 555555555575555555555,55555555555755555555555,5555555555557555555555555,555555555555575555555555555,55555555555555755555555555555,5555555555555557555555555555555
%N A332157 a(n) = 5*(10^(2*n+1)-1)/9 + 2*10^n.
%H A332157 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332157 a(n) = 5*A138148(n) + 7*10^n = A002279(2n+1) + 2*10^n.
%F A332157 G.f.: (7 - 202*x - 300*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332157 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332157 A332157 := n -> 5*(10^(2*n+1)-1)/9+2*10^n;
%t A332157 Array[5 (10^(2 # + 1)-1)/9 + 2*10^# &, 15, 0]
%o A332157 (PARI) apply( {A332157(n)=10^(n*2+1)\9*5+2*10^n}, [0..15])
%o A332157 (Python) def A332157(n): return 10**(n*2+1)//9*5+2*10**n
%Y A332157 Cf. A002275 (repunits R_n = (10^n-1)/9), A002279 (5*R_n), A011557 (10^n).
%Y A332157 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332157 Cf. A332117 .. A332197 (variants with different repeated digit 1, ..., 9).
%Y A332157 Cf. A332150 .. A332159 (variants with different middle digit 0, ..., 9).
%K A332157 nonn,base,easy
%O A332157 0,1
%A A332157 _M. F. Hasler_, Feb 09 2020