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a(1)=0; thereafter a(n) = 2^a(ceiling(n/2)).
2

%I #7 Nov 07 2013 11:57:46

%S 0,1,2,2,4,4,4,4,16,16,16,16,16,16,16,16,65536,65536,65536,65536,

%T 65536,65536,65536,65536,65536,65536,65536,65536,65536,65536,65536,

%U 65536

%N a(1)=0; thereafter a(n) = 2^a(ceiling(n/2)).

%C a(33) through a(64) are 2^65536.

%F For n>2, a(n) = a tower of 2's of height ceiling(log_2(n))-1. E.g. a(17) = 2^2^2^2.

%Y Cf. A230863, A230874.

%K nonn

%O 1,3

%A _N. J. A. Sloane_, Nov 07 2013