What year did Barack Obama's mother give up US citizenship?

Barack Obama, the president, never had Kenyan citizenship because he was born in the USA (in Hawaii) and is an American citizen. Barack Obama Sr. (his dad) never gave up his Kenyan citizenship, even though he lived in the USA and studied here for a number of years. (This is a "Birther" question-- it comes from a group of Republicans who still do not believe President Obama was born in the USA, and who keep asking about Kenya. It has been well-documented that President Obama never was born in Kenya and did not even visit Kenya till the late 1980s.)