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I thought the issue was the technicality about a person's parent/s having to live in the USA for a certain number of years and half of those after the age of 16. Apparently, at the time of Obama's birth, his mother was too young to have fulfilled that criteria.
You're correct in saying that a in order to transmit citizenship, the parent must have spent 5 years in the U.S. after the age of 16. Clearly, Obama's mother met this standard. Of course, this is all irrelevant because Obama was born in the United States!