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a(0) = 1, a(n)= a(n-1)[n-1]a(n-1).
1

%I #7 Jan 11 2020 15:57:47

%S 1,2,4,16,18446744073709551616

%N a(0) = 1, a(n)= a(n-1)[n-1]a(n-1).

%C a[n]b is the square-bracket notation for n-th hyperoperation of a times b see A054871 for more info.

%e a(0) = 1;

%e a(1) = a(0)[0]a(0) = 1’= 2;

%e a(2) = a(1)[1]a(1) = 2+2 = 4;

%e a(3) = a(2)[2]a(2) = 4*4 = 16;

%e a(4) = a(3)[3]a(3) = 16^16 = 18446744073709551616.

%Y Cf. A271551.

%K nonn

%O 0,2

%A _Natan Arie Consigli_, Apr 09 2016