This is Wilder’s Smoothing Moving Average (WSMA) AKA Smoothed Moving Average indicator.
The first Value is a Simple Moving Average (SMA) and All Subsequent values are calculated based on the Previous value according to the following formula :
SUM(1) = SUM(CLOSE, N)
WSMA(1) = Simple MA = SUM(1)/N - Wilder’s Smoothing for the first period.
WSMA(i) = (SUM(i - 1) - WSMA(i - 1) + CLOSE(i)) / N
