I asked GPT-4

The statement you read is not entirely accurate. The complexity of Bitcoin's Proof of Work (PoW) function is not "O(2^(log n) / n)". Instead, the complexity of finding a valid PoW solution can be expressed as "O(2^k)", where "k" is the number of leading zeros required in the hash output, which is determined by the target difficulty.

O(): The big-O notation, which describes the upper bound of the growth rate of an algorithm. It is used to analyze and compare the efficiency of algorithms in terms of their time complexity or the number of operations they perform.