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A332178 a(n) = 7*(10^(2n+1)-1)/9 + 10^n.

%I A332178 #8 Feb 11 2020 08:26:47
%S A332178 8,787,77877,7778777,777787777,77777877777,7777778777777,
%T A332178 777777787777777,77777777877777777,7777777778777777777,
%U A332178 777777777787777777777,77777777777877777777777,7777777777778777777777777,777777777777787777777777777,77777777777777877777777777777,7777777777777778777777777777777
%N A332178 a(n) = 7*(10^(2n+1)-1)/9 + 10^n.
%C A332178 See A183182 = {1, 3, 39, 54, 168, 240, ...} for the indices of primes.
%H A332178 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77877.htm">Factorization of 77...77877...77</a>, updated Dec 11 2018.
%H A332178 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332178 a(n) = 7*A138148(n) + 8*10^n.
%F A332178 G.f.: (8 - 101*x - 600*x^2)/((1 - x)*(1 - 10*x)*(1 - 100*x)).
%F A332178 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332178 A332178 := n -> 7*(10^(n*2+1)-1)/9 + 10^n;
%t A332178 Array[7 (10^(2 # + 1) - 1)/9 + 10^# &, 15, 0]
%o A332178 (PARI) apply( {A332178(n)=10^(n*2+1)\9*7+10^n}, [0..15])
%o A332178 (Python) def A332178(n): return 10**(n*2+1)//9*7+10^n
%Y A332178 Cf. A138148 (cyclops numbers with binary digits only).
%Y A332178 Cf. (A077793-1)/2 = A183182: indices of primes.
%Y A332178 Cf. A002275 (repunits R_n = (10^n-1)/9), A002281 (7*R_n), A011557 (10^n).
%Y A332178 Cf. A332171 .. A332179 (variants with different middle digit 1, ..., 9).
%K A332178 nonn,base,easy
%O A332178 0,1
%A A332178 _M. F. Hasler_, Feb 08 2020