The first terms are (see Wikipedia):
a(0) = 2^2^2 + 2^1 + 2^0 = 19
a(1) = 3^3^3 + 3^1 + 3^0 - 1 = 7625597484990
a(2) = 4^4^4 + 4^1 - 1 (155 digits)
a(3) = 5^5^5 + 3 - 1 (2185 digits)
a(4) = 6^6^6 + 2 - 1 (36306 digits)
a(5) = 7^7^7 + 1 - 1 (695975 digits)
a(6) = 8^8^8 - 1 (15151336 digits).
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