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%I #7 Feb 11 2020 07:59:13
%S 0,202,22022,2220222,222202222,22222022222,2222220222222,
%T 222222202222222,22222222022222222,2222222220222222222,
%U 222222222202222222222,22222222222022222222222,2222222222220222222222222,222222222222202222222222222,22222222222222022222222222222,2222222222222220222222222222222
%N a(n) = 2*(10^(2n+1)-1)/9 - 2*10^n.
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F a(n) = 2*A138148(n) = A002276(2n+1) - 2*10^n.
%F G.f.: 2*x*(101 - 200*x)/((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 A332120 := n -> 2*((10^(2*n+1)-1)/9-10^n);
%t Array[2 ((10^(2 # + 1)-1)/9 - 10^#) &, 15, 0]
%o (PARI) apply( {A332120(n)=(10^(n*2+1)\9-10^n)*2}, [0..15])
%o (Python) def A332120(n): return (10**(n*2+1)//9-10**n)*2
%Y Cf. A002275 (repunits R_n = (10^n-1)/9), A002276 (2*R_n), A011557 (10^n).
%Y Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y Cf. A332130 .. A332190 (variants with different repeated digit 3, ..., 9).
%Y Cf. A332121 .. A332129 (variants with different middle digit 1, ..., 9).
%K nonn,base,easy
%O 0,2
%A _M. F. Hasler_, Feb 09 2020
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