G. Athanasopoulos, R. A. Ahmed and R. J. Hyndman (2009) Hierarchical forecasts for Australian domestic tourism, International Journal of Forecasting, 25, 146-166.R. J. Hyndman, R. A. Ahmed, G. Athanasopoulos and H.L. Shang (2011)
Optimal combination forecasts for hierarchical time series.
Computational Statistics and Data Analysis, 55(9), 2579--2589.
http://robjhyndman.com/papers/hierarchical/