I am a university retired professor in Brasil . I pay taxes in Brasil discount in the source.
Shall I pay taxes also IN USA

A U.S. citizen or resident must report worldwide income even when he or she does not live in the U.S. If your total worldwide income exceeds the filing requirement, a U.S. citizen or resident must file your U.S. tax return. If you paid taxes in the foreign country, you will claim credit for taxes paid in the foreign country by filing Form 1116 or can file form 2555 Foreign Earned Income Exclusion. Even if you have a filing requirement, the chances are you will not pay any U.S. tax or pay very little U.S. tax.
Another filing requirement is Form TD F 90-22.1. Your U.S. Tax Return: U.S. Citizen or Resident with Foreign Income (http://taxipay.blogspot.com/2008/03/us-citizen-or-resident-with-foreign.html)