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by o11c 501 days ago
Relevant link: https://en.wikipedia.org/wiki/Change-making_problem

Looking at cases where greedy isn't optimal, I see two patterns. Either:

* there are two coins close in value (ratio of less than 2), e.g. both 20¢ and 25¢; or

* there is a "missing" coin at the GCD of two larger coins.

I'm pretty sure you can precompute extra "virtual coins" (e.g. 40¢) to make greedy optimal again, but you have to place restrictions on how many of them you're allowed to use.