%I #24 Oct 15 2021 16:57:39
%S 1,3,8,18,39,85,189,422,942,2099,4673,10400,23148,51528,114709,255359,
%T 568460,1265450,2817015,6270953,13959773,31075874,69178058,153997383,
%U 342813793,763138256,1698823128,3781752544,8418564665,18740578667,41718428560,92869452514
%N a(n) = 1 + 2 * (a(n-1) + a(n-4) + a(n-6)) + a(n-7) for n>3, with initial values 0 if n<0, and 1,3,8,18 for n=0..3.
%C Some mail programs automatically include copies of all earlier messages in the same thread. In some cases (especially when rich text format is used) any unrecognized character is replaced by two copies of itself, leading to exponential growth.
%C The present sequence illustrates this growth for UTF-8 encoding and the CP1252 character set, for the single code point 150.
%D Fred Lunnon and Tomas Rokicki, Posting to Math Fun Mailing List, circa Jul 25 2018
%H Harvey P. Dale, <a href="/A317188/b317188.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_08">Index entries for linear recurrences with constant coefficients</a>, signature (3,-2,0,2,-2,2,-1,-1).
%F G.f.: -(x^4-x^2-1)/((x-1)*(x+1)*(x^6+x^5-x^4+3*x^3-3*x^2+3*x-1)). - _Alois P. Heinz_, Jul 28 2018
%t LinearRecurrence[{3,-2,0,2,-2,2,-1,-1},{1,3,8,18,39,85,189,422},40] (* _Harvey P. Dale_, Oct 15 2021 *)
%K nonn,easy
%O 0,2
%A _Tomas Rokicki_, Jul 28 2018
%E More terms from _Altug Alkan_, Jul 28 2018