Butterworth Filter script.
This indicator was described by John F. Ehlers in his book "Rocket Science for Traders" (2001, Chapter 15: Infinite Impulse Response Filters).
Infinite Impulse Response (IIR) Filter indicator script.
This indicator was originally developed by John Ehlers (Stocks & Commodities V. 20:7 (26-31): Zero-Lag Data Smoothers).
