Ackermann number

An Ackermann Number is a number that can be expressed as $$n\underbrace{\uparrow\uparrow...\uparrow\uparrow}_nn$$ (using Arrow Notation), where n is a positive integer. The first few Ackermann numbers are $$1\uparrow 1 = 1$$, $$2\uparrow\uparrow 2 = 4$$, and $$3\uparrow\uparrow\uparrow 3 =$$ tritri.