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 A332147 #10 Mar 08 2022 15:04:00
%S A332147 7,474,44744,4447444,444474444,44444744444,4444447444444,
%T A332147 444444474444444,44444444744444444,4444444447444444444,
%U A332147 444444444474444444444,44444444444744444444444,4444444444447444444444444,444444444444474444444444444,44444444444444744444444444444,4444444444444447444444444444444
%N A332147 a(n) = 4*(10^(2*n+1)-1)/9 + 3*10^n.
%H A332147 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332147 a(n) = 4*A138148(n) + 7*10^n = A002278(2n+1) + 3*10^n.
%F A332147 G.f.: (7 - 303*x - 100*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332147 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332147 A332147 := n -> 4*(10^(2*n+1)-1)/9+3*10^n;
%t A332147 Array[4 (10^(2 # + 1)-1)/9 + 3*10^# &, 15, 0]
%t A332147 LinearRecurrence[{111,-1110,1000},{7,474,44744},20] (* _Harvey P. Dale_, Mar 08 2022 *)
%o A332147 (PARI) apply( {A332147(n)=10^(n*2+1)\9*4+3*10^n}, [0..15])
%o A332147 (Python) def A332147(n): return 10**(n*2+1)//9*4+3*10**n
%Y A332147 Cf. A002275 (repunits R_n = (10^n-1)/9), A002278 (4*R_n), A011557 (10^n).
%Y A332147 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332147 Cf. A332117 .. A332197 (variants with different repeated digit 1, ..., 9).
%Y A332147 Cf. A332140 .. A332149 (variants with different middle digit 0, ..., 9).
%K A332147 nonn,base,easy
%O A332147 0,1
%A A332147 _M. F. Hasler_, Feb 09 2020