Description of Aroon oscillator: http://www.investopedia.com/terms/a/aroon.asp.
Formulas:
Aroon Up[i] = 100*(Length-i+PosMax)/Length,
Aroon Dn[i] = 100*(Length-i+PosMin)/Length, where
PosMax, PosMin - positions (number of bar) of maximum and minimum prices at range from (i-Length+1) to (i).
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