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 A332176 #8 Feb 11 2020 08:26:44
%S A332176 6,767,77677,7776777,777767777,77777677777,7777776777777,
%T A332176 777777767777777,77777777677777777,7777777776777777777,
%U A332176 777777777767777777777,77777777777677777777777,7777777777776777777777777,777777777777767777777777777,77777777777777677777777777777,7777777777777776777777777777777
%N A332176 a(n) = 7*(10^(2n+1)-1)/9 - 10^n.
%C A332176 See A183181 = {4, 5, 8, 11, 1244, 1685, ...} for the indices of primes.
%H A332176 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332176 a(n) = 7*A138148(n) + 6*10^n.
%F A332176 G.f.: (6 + 101*x - 800*x^2)/((1 - x)*(1 - 10*x)*(1 - 100*x)).
%F A332176 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332176 A332176 := n -> 7*(10^(n*2+1)-1)/9 - 10^n;
%t A332176 Array[7 (10^(2 # + 1) - 1)/9 - 10^# &, 15, 0]
%o A332176 (PARI) apply( {A332176(n)=10^(n*2+1)\9*7-10^n}, [0..15])
%o A332176 (Python) def A332176(n): return 10**(n*2+1)//9*7-10^n
%Y A332176 Cf. A138148 (cyclops numbers with binary digits only).
%Y A332176 Cf. (A077788-1)/2 = A183181: indices of primes.
%Y A332176 Cf. A002275 (repunits R_n = (10^n-1)/9), A002281 (7*R_n), A011557 (10^n).
%Y A332176 Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 9).
%K A332176 nonn,base,easy
%O A332176 0,1
%A A332176 _M. F. Hasler_, Feb 08 2020