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 A332172 #9 Feb 11 2020 08:26:33
%S A332172 2,727,77277,7772777,777727777,77777277777,7777772777777,
%T A332172 777777727777777,77777777277777777,7777777772777777777,
%U A332172 777777777727777777777,77777777777277777777777,7777777777772777777777777,777777777777727777777777777,77777777777777277777777777777,7777777777777772777777777777777
%N A332172 a(n) = 7*(10^(2n+1)-1)/9 - 5*10^n.
%C A332172 Indices of prime terms: {0, 1, 3, 7, 10, 12, 480, 949, ...} = A183178.
%H A332172 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332172 a(n) = 7*A138148(n) + 2*10^n.
%F A332172 G.f.: (2 + 505*x - 1200*x^2) / ((1 - x)*(1 - 10*x)*(1 - 100*x)).
%F A332172 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n>2.
%p A332172 A332172 := n -> 7*(10^(n*2+1)-1)/9 -5*10^n;
%t A332172 Array[7 (10^(2 # +1)-1)/9 -5*10^# &, 15, 0]
%o A332172 (PARI) apply( {A332172(n)=10^(n*2+1)\9*7-5*10^n}, [0..25])
%o A332172 (Python) def A332172(n): return 10**(n*2+1)//9*7-5*10^n
%Y A332172 Cf. A138148 (cyclops numbers with binary digits only).
%Y A332172 Cf. A332171 (analog with middle digit 1).
%Y A332172 Cf. (A077777-1)/2 = A183178: indices of primes.
%Y A332172 Cf. A002275 (repunits R_n = [10^n/9]), A002281 (7*R_n), A011557 (10^n).
%Y A332172 Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 9).
%K A332172 nonn,base,easy
%O A332172 0,1
%A A332172 _M. F. Hasler_, Feb 06 2020