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 A332170 #9 Feb 11 2020 08:24:27
%S A332170 0,707,77077,7770777,777707777,77777077777,7777770777777,
%T A332170 777777707777777,77777777077777777,7777777770777777777,
%U A332170 777777777707777777777,77777777777077777777777,7777777777770777777777777,777777777777707777777777777,77777777777777077777777777777,7777777777777770777777777777777
%N A332170 a(n) = 7*(10^(2n+1)-1)/9 - 7*10^n.
%H A332170 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332170 a(n) = 7*A138148(n) = A002281(2n+1) - 7*A011557(n).
%F A332170 G.f.: 7*x*(101 - 200*x)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332170 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332170 A332170 := n -> 7*(10^(2*n+1)-1)/9-7*10^n;
%t A332170 Array[7 ((10^(2 # + 1)-1)/9 - 10^#) &, 15, 0]
%o A332170 (PARI) apply( {A332170(n)=(10^(n*2+1)\9-10^n)*7}, [0..15])
%o A332170 (Python) def A332170(n): return (10**(n*2+1)//9-10^n)*7
%Y A332170 Cf. A002275 (repunits R_n = (10^n-1)/9), A002281 (7*R_n), A011557 (10^n).
%Y A332170 Cf. A138148 (cyclops numbers with binary digits only), A002113 (palindromes).
%Y A332170 Cf. A332120 .. A332190 (variants with different repeated digit 2, ..., 9).
%Y A332170 Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 9).
%K A332170 nonn,base,easy
%O A332170 0,2
%A A332170 _M. F. Hasler_, Feb 08 2020