* Brent crude flat on day but down 4 pct on week * Copper up for both session and week * Gold rises for the day but ends week lower * Corn at 2-1/2 week low, biggest fall in almost a month * Arabica up on short covering, rising from 2-1/2 year low By Barani Krishnan NEW YORK, Dec 7 (Reuters) - Oil seesawed before closing lower on Friday and sharply lower for the week, while copper rose on the day as stronger-than-expected U.S. jobs growth in November offset deadlocked ta