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%I #6 Feb 11 2020 08:26:50
%S 9,797,77977,7779777,777797777,77777977777,7777779777777,
%T 777777797777777,77777777977777777,7777777779777777777,
%U 777777777797777777777,77777777777977777777777,7777777777779777777777777,777777777777797777777777777,77777777777777977777777777777,7777777777777779777777777777777
%N a(n) = 7*(10^(2n+1)-1)/9 + 2*10^n.
%C See A183183 = {1, 2, 8, 19, 20, 212, 280, ...} for the indices of primes.
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F a(n) = 7*A138148(n) + 9*10^n.
%F G.f.: (9 - 202*x - 500*x^2)/((1 - x)*(1 - 10*x)*(1 - 100*x)).
%F a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332179 := n -> 7*(10^(n*2+1)-1)/9 + 2*10^n;
%t Array[7 (10^(2 # + 1) - 1)/9 + 2*10^# &, 15, 0]
%o (PARI) apply( {A332179(n)=10^(n*2+1)\9*7+2*10^n}, [0..15])
%o (Python) def A332179(n): return 10**(n*2+1)//9*7+2*10^n
%Y Cf. A138148 (cyclops numbers with binary digits only).
%Y Cf. (A077796-1)/2 = A183183: indices of primes.
%Y Cf. A002275 (repunits R_n = [10^n/9]), A002281 (7*R_n), A011557 (10^n).
%Y Cf. A332171 .. A332178 (variants with different middle digit 1, ..., 8).
%K nonn,base,easy
%O 0,1
%A _M. F. Hasler_, Feb 08 2020
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