(Portland, OR)  --  Portland police are looking for a sex offender who's described as having a history of violent behavior.  Authorities say 56-year-old Dennis John Davis, who's wanted for a parole violation, removed his electronic monitoring device after he was released from prison Friday.  Davis is described as white, five-feet-nine, 197 pounds, with gray hair and blue eyes, and police say he may have shaved his head.  Anyone seeing Davis is asked to call 911.