If we let $z=xy$, we have: $$z=\sum_{i=1}^{y}x$$ So multiplication can be written as iterative sum. Likewise, if we have $z=x^y$, then we can write: $$z=\prod_{i=1}^{y}x$$ But how to write the exponent in sum form instead of product form?
I tried to experiment by writing for example $5^2=(5+5+5+5+5), 2^4=((2+2)+(2+2))+((2+2)+(2+2))$, but I came up with nothing.
Edit: From the conversation in the comments, you can write exponent as nested summations:
$$z=\sum_{i=1}^{x}\sum_{j=1}^{x}\sum_{k=1}^{x}\dots\text{(y times)}\dots x$$