cp's OEIS Frontend

A278449 a(n) = nearest integer to b(n) = c^(b(n-1)/(n-1)), where c=3 and b(1) is chosen such that the sequence neither explodes nor goes to 1.

%I A278449 #21 Dec 02 2016 00:15:33
%S A278449 1,3,6,9,13,16,20,24,29,33,37,42,47,51,56,61,66,71,76,81,86,92,97,102,
%T A278449 108,113,118,124,129,135,141,146,152,158,163,169,175,181,187,193,199,
%U A278449 205,210,216,222,229,235,241,247,253,259,265,271,278,284,290,296,303,309,315,322,328,334,341,347,354,360,367,373,379
%N A278449 a(n) = nearest integer to b(n) = c^(b(n-1)/(n-1)), where c=3 and b(1) is chosen such that the sequence neither explodes nor goes to 1.
%C A278449 For the given c there exists a unique b(1) for which the sequence b(n) does not converge to 1 and at the same time always satisfies b(n-1)b(n+1)/b(n)^2 < 1 (due to rounding to the nearest integer a(n-1)a(n+1)/a(n)^2 is not always less than 1).
%C A278449 In this case b(1) = 1.0828736095... A278809. If b(1) were chosen smaller the sequence would approach 1, if it were chosen greater the sequence would at some point violate b(n-1)b(n+1)/b(n)^2 < 1 and from there on quickly escalate.
%C A278449 The value of b(1) is found through trial and error. Illustrative example for the case of c=2 (for c=3 similar): "Suppose one starts with b(1) = 2, the sequence would continue b(2) = 4, b(3) = 4, b(4) = 2.51..., b(5) = 1.54... and from there one can see that such a sequence is tending to 1. One continues by trying a larger value, say b(1) = 3, which gives rise to b(2) = 8, b(3) = 16, b(4) = 40.31... and from there one can see that such a sequence is escalating too fast. Therefore, one now knows that the true value of b(1) is between 2 and 3."
%C A278449 b(n) = n*log_3((n+1)*log_3((n+2)*log_3(...))) ~ n*log_3(n). - _Andrey Zabolotskiy_, Dec 01 2016
%H A278449 Rok Cestnik, <a href="/A278449/b278449.txt">Table of n, a(n) for n = 1..1000</a>
%H A278449 Rok Cestnik, <a href="/A278449/a278449.pdf">Plot of the dependence of b(1) on c</a>
%e A278449 a(2) = round(3^1.08...) = round(3.28...) = 3.
%e A278449 a(3) = round(3^(3.28.../2)) = round(6.07...) = 6.
%e A278449 a(4) = round(3^(6.07.../3)) = round(9.26...) = 9.
%t A278449 c = 3;
%t A278449 n = 100;
%t A278449 acc = Round[n*1.2];
%t A278449 th = 1000000;
%t A278449 b1 = 0;
%t A278449 For[p = 0, p < acc, ++p,
%t A278449   For[d = 0, d < 9, ++d,
%t A278449     b1 = b1 + 1/10^p;
%t A278449     bn = b1;
%t A278449     For[i = 1, i < Round[n*1.2], ++i,
%t A278449      bn = N[c^(bn/i), acc];
%t A278449      If[bn > th, Break[]];
%t A278449      ];
%t A278449     If[bn > th, {
%t A278449       b1 = b1 - 1/10^p;
%t A278449       Break[];
%t A278449       }];
%t A278449     ];
%t A278449   ];
%t A278449 bnlist = {N[b1]};
%t A278449 bn = b1;
%t A278449 For[i = 1, i < n, ++i,
%t A278449   bn = N[c^(bn/i), acc];
%t A278449   If[bn > th, Break[]];
%t A278449   bnlist = Append[bnlist, N[bn]];
%t A278449   ];
%t A278449 anlist = Map[Round[#] &, bnlist]
%Y A278449 For decimal expansion of b(1) see A278809.
%Y A278449 For different values of c see A278448, A278450, A278451, A278452.
%Y A278449 For b(1)=0 see A278453.
%K A278449 nonn
%O A278449 1,2
%A A278449 _Rok Cestnik_, Nov 22 2016