The stock markets remained upbeat during the week ended September 7 as buying gathered momentum on the back of robust economic growth, easing inflation and the return of foreign institutional investors (FIIs).The Bombay Stock Exchange BSE index, Sensex, moved between a high of 15,716.06 and low of 15,323.05 before ending the week at 15,590.42 - a net rise of 271.82 points (1.77%) over the last weekend close of 15,318.60.The broad-based S&P CNX Nifty of the National Stock Exchange (NSE) gained 45.50 points (1.02%) to close at 4,509.50 as against the last weekend close of 4,464.00.The Sensex has gained 1,448.89 points (10.25%) in the last three weeks (August 20-September 7).Foreign institutional investors stepped up activity in the week after their massive pull-out during August. They pumped in about Rs 2,576 crore (including the provisional numbers of Friday) in the week.Inflation dropped to a 16-month low of 3.79% for the week ended August 25 due to the decline in prices of food articles.