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%I #44 Dec 27 2021 22:16:04
%S 0,1,3,27,7625597484987
%N a(0) = 0; thereafter a(n+1) = 3^a(n).
%C Alternative definition:
%C a(n+1) = H_4(3,n) the tetration (repeated exponentiation) of 3 times n.
%C For definition of and references concerning H_n(x,y) see A054871.
%C Next term is 12580...39387 and has 3638334640025 digits. - Shawn Ligocki (sligocki(AT)gmail.com), Oct 08 2009
%F a(n+1) = H_4(3,n) = 3^^n;
%F a(0) = 0, a(1) = 1, a(n+1) = 3^3^...^3 (n times).
%e a(0) = H_4(3,-1)= 0;
%e a(1) = H_4(3,0) = 1;
%e a(2) = H_4(3,1) = 3;
%e a(3) = H_4(3,2) = 3^3 = 27;
%e a(4) = H_4(3,3) = 3^3^3 = 7625597484987;
%e a(5) = H_4(3,4) = 3^3^3^3 = 3^7625597484987 > 10^3638334640025.
%t NestList[3^#&,0,4] (* _Harvey P. Dale_, Apr 04 2013 *)
%Y Cf. A014221 (a(n+1) = 2^a(n), or H_4(2,n)), A081651, A114561, A266199 (H_5(3,n)).
%K nonn,easy
%O 0,3
%A _N. J. A. Sloane_
%E Revised using hyperoperation notation by _Natan Arie Consigli_, Jan 16 2016
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