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$$
3^{(3^3)} = 3^{(27)}  = 7625597484987\quad\quad (3^3)^3 = 27^3 = 19683
$$
 
The difference rapidly grows for larger values:
 $$ 4^{(4^4)} = 4^{(256)} \sim 10^{154} \quad\quad (4^4)^4 = 256^4\sim 10^9 $$
 
However, for $2$ the values are the same
 
 $$ 2^{(2^2)} = 2^{(4)} = 16\quad\quad (2^2)^2 =4^2 =16 $$
 
The extension of the definitions are naturally either 'powers evaluated from the right' or 'powers evaluated from the left'. The difference for a stack of four powers is gigantic
 
$$
(((3^3)^3)^3) = (((27)^3)^3) = (19683)^3\sim 10^{12}
$$

$$
(3^{(3^{(3^{(3)})})}) =(3^{(3^{27})}) =(3^{(7.6\times 10^{12})})\sim 10^{3.6\times 10^{12}}
$$
 
Using a spreadsheet we found that both definition of stacking four numbers leads to the same value when the base is $1.02092370325178$
 

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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