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a(n) = (a(n-1)+1)^(a(n-1)), a(0) = 0.
15

%I #15 Aug 13 2020 19:22:20

%S 0,1,2,9,1000000000

%N a(n) = (a(n-1)+1)^(a(n-1)), a(0) = 0.

%C Let S(1) c S(2) c ... c S(n) c ... be an increasing sequence of sets of partial functions that is defined as follows: S(0) = empty set, S(n) = {partial functions: S(n-1) -> S(n-1)}. Then |S(n)| = a(n). - _Jon Awbrey_, Jul 04 2005

%H J. Awbrey, <a href="https://oeis.org/wiki/Riffs_and_Rotes">Riffs and Rotes</a>

%t NestList[(#+1)^#&,0,4] (* _Harvey P. Dale_, Aug 13 2020 *)

%Y Cf. A109300, A109301.

%K easy,nonn

%O 0,3

%A Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 30 1999

%E The next term is approximately e * 10^9000000000, with nine-place accuracy. - _Franklin T. Adams-Watters_, Nov 16 2006

%E a(5) = 2.7182818270999043223766*10^9000000000 = e * 10^9000000000 * 0.9999999995000000004583. - _Jon E. Schoenfield_, Nov 24 2013