If I post what is needed to do in the challenge and provide multiple
input -> output example, would it be clear enough?
Only for test batteries: if the inputs listed in the spec are the only possible inputs, there's no need to explain the pattern.
Unless the pattern is extremely simple and obvious, people should not have to try and guess the pattern from the test cases. Chances are, the test cases do not cover all edge cases, leaving the challenge ambiguous and invalidating answers when further test cases are added to clarify it.
The problem is that I don't know how to explain it using technical words or mathematical expressions.
That means the pattern is complex and in need of an explanation. If the challenge doesn't contain one, it will most likely get closed as unclear what you're asking. I would vote to close myself.
However, you can absolutely post your challenge to the sandbox and get the community's help to come up with a text explanation. That's what the sandbox is for.