In trading on Thursday, shares of the First Trust Mid Cap Core AlphaDEX Fund ETF ( FNX) entered into oversold territory, changing hands as low as $50.27 per share. We define oversold territory using the Relative Strength Index, or RSI, which is a technical analysis indicator used to measure momentum on a scale of zero to 100. A stock is considered to be oversold if the RSI reading falls below 30.In the case of First Trust Mid Cap Core AlphaDEX Fund, the RSI reading has hit 29.5 — by comparison, the RSI reading for the S&P 500 is currently 32.2. A bullish investor could look at FNX's 29.5 reading as a sign that the recent heavy selling is in the process of exhausting itself, and begin to look for entry point opportunities on the buy side. Looking at a chart of one year performance (below), FNX's low point in its 52 week range is $42.91 per share, with $55.03 as the 52 week high point — that compares with a last trade of $50.31. First Trust Mid Cap Core AlphaDEX Fund shares are currently trading off about 0.6% on the day.