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a(n) = 4*(10^n-5).
1

%I #18 Jul 26 2024 13:14:13

%S 20,380,3980,39980,399980,3999980,39999980,399999980,3999999980,

%T 39999999980,399999999980,3999999999980,39999999999980,

%U 399999999999980,3999999999999980,39999999999999980,399999999999999980

%N a(n) = 4*(10^n-5).

%C Is this 20*A067272(n)?

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).

%F G.f.: 20*x*(1+8*x) / ( (10*x-1)*(x-1) ). - _R. J. Mathar_, Jan 06 2011

%F a(n) = 11*a(n-1) - 10*a(n-2). - _Wesley Ivan Hurt_, Mar 17 2023

%p A177109:=n->4*(10^n-5): seq(A177109(n), n=1..30); # _Wesley Ivan Hurt_, Feb 16 2017

%t 4(10^Range[20]-5) (* or *) LinearRecurrence[{11,-10},{20,380},20] (* _Harvey P. Dale_, Jul 26 2024 *)

%Y Cf. A067272.

%K nonn,easy

%O 1,1

%A _Vincenzo Librandi_, Nov 15 2010