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%I #26 Jun 29 2023 18:44:51
%S 0,800,98000,9980000,999800000,99998000000,9999980000000,
%T 999999800000000,99999998000000000,9999999980000000000,
%U 999999999800000000000,99999999998000000000000,9999999999980000000000000,999999999999800000000000000,99999999999998000000000000000
%N a(n) is the largest number such that all of a(n)'s length-n substrings are distinct and divisible by 20.
%C For n > 1, a(n) is n-2 9's followed by an 8, followed by n 0's. - _John Cerkan_, May 23 2017
%H Hans Havermann, <a href="/A093211/a093211.txt">Table of A093211-A093299</a>
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (10, -20).
%F From _Chai Wah Wu_, Oct 25 2018: (Start)
%F a(n) = 110*a(n-1) - 1000*a(n-2) for n > 3.
%F G.f.: 400*x^2*(25*x + 2)/((10*x - 1)*(100*x - 1)). (End)
%e a(3) is 98000 because its length-3 substrings (980, 800, 000) are all distinct and divisible by 20 and there is no larger number with this property
%o (GAP) a:=[800,98000];; for n in [3..12] do a[n]:=110*a[n-1]-1000*a[n-2]; od; Concatenation([0],a); # _Muniru A Asiru_, Oct 25 2018
%o (PARI) concat(0, Vec(400*x^2*(25*x + 2)/((10*x - 1)*(100*x - 1)) + O(x^20))) \\ _Felix Fröhlich_, Oct 25 2018
%Y Cf. A093211, A093212, ..., A093299.
%K base,nonn
%O 1,2
%A _Hans Havermann_, Mar 28 2004
%E a(8)-a(10) from _John Cerkan_, May 23 2017
%E More terms from _Felix Fröhlich_, Oct 25 2018