Two crucial sources for time series analysis differ in a critical formula for equivalence between simple exponential smoothing (SES) and ARIMA(0, 1, 1).
From Hyndman's F:PP:
$\theta_1 = \alpha - 1$
and from R. Nau's Statistical Forecasting:
$\theta_1 = 1- \alpha$
Can someone help me understand the apparent discrepancy?