The true strength index (TSI) is a technical indicator used in the analysis of financial markets that attempts to show both trend direction and overbought/oversold conditions. It was first published William Blau in 1991. The indicator uses moving averages of the underlying momentum of a financial instrument. Momentum is considered a leading indicator of price movements, and a moving average characteristically lags behind price. The TSI combines these characteristics to create an indication of price and direction more in sync with market turns than either momentum or moving average. (source: wikipedia)
a = (ExponentialAverage[13](ExponentialAverage[25](ROC[1](close))))
b = (ExponentialAverage[13](ExponentialAverage[25](ABS(ROC[1](close)))))
TSI = 100 * (a/b)
RETURN TSI as "True Strength Index", 0