Can I ask dafny to not check termination of a function?


Functions must always terminate, and it must be proved that they terminate.

Methods on the other hand should also terminate , but you can request the the proof be skipped using decreases *, as in

method inf(i: int)
    decreases *

Eventually you should put an actual termination metric in place of the * and prove termination. The reference manual has more information about termination metrics in the section on decreases clauses.