%I #38 Sep 08 2022 08:46:09
%S 0,609,66099,6660999,666609999,66666099999,6666660999999,
%T 666666609999999,66666666099999999,6666666660999999999,
%U 666666666609999999999,66666666666099999999999,6666666666660999999999999,666666666666609999999999999,66666666666666099999999999999
%N 6*((10^n-1)/9)*(10^(n+1))+9*(10^n-1)/9.
%C Numbers of the form 6...609...9 (i.e., consisting of an odd number of digits with the middle digit 0, all digits to the left of the middle digit 6 and all digits to the right of the middle digit 9).
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F G.f.: 20/(3-300*x)+1/(x-1)+17/(30*x-3); a(n) = 111*a(n-1)-1110*a(n-2)+1000*a(n-3). - _Wesley Ivan Hurt_, Sep 15 2014
%p A246880:=n->(6*((10^n - 1)/9))*(10^(n + 1)) + (9*(10^n - 1)/9): seq(A246880(n),n=0..15); # _Wesley Ivan Hurt_, Sep 15 2014
%t Table[(6*((10^n - 1)/9))*(10^(n + 1)) + (9*(10^n - 1)/9), {n, 15}] (* _Wesley Ivan Hurt_, Sep 15 2014 *)
%t Join[{0}, CoefficientList[Series[20/(3 x - 300 x^2) + 1/(x^2 - x) + 17/(30 x^2 - 3 x), {x, 0, 30}], x]] (* _Wesley Ivan Hurt_, Sep 15 2014 *)
%o (PARI) a(n)=6*((10^n-1)/9)*(10^(n+1))+9*(10^n-1)/9
%o (Magma) [(6*((10^n - 1)/9))*(10^(n + 1)) + (9*(10^n - 1)/9) : n in [0..15]]; // _Wesley Ivan Hurt_, Sep 15 2014
%Y Subsequence of A001743, A039434, A111065, A111156, A153806, A169731 and A227793.
%K nonn,base,easy
%O 0,2
%A _Felix Fröhlich_, Sep 06 2014
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