Non-moderators can only vote to migrate questions to a small set of predefined sites. On Math.SE, the set of potential migration targets consists of Physics.SE and Stats.SE (plus meta).
Stack Exchange staff can add migration targets (up to 5, but few sites have that many). As a matter of general policy, beta sites are not added as migration targets.
Moderators can migrate a question to any site. The only constraint is that the question must be less than 60 days old; even moderators cannot migrate other questions. If you see a question that is off-topic on Math but on-topic on CS and less than 60 days old, flag the question and request the question to be migrated.
https://math.stackexchange.com/questions/1220665/graph-diameter-approximation is not a good question and should not be migrated. Although Computer Science SE allows homework questions, we don't like problem dumps like this, and generally close them as unclear what you're asking — we want the asker to tell us what they tried and where they got stuck so that we can help them with the part that caused them trouble.
The boundary between math (on-topic) and theoretical computer science (off-topic) is a separate issue.