text: * Roberto Todeschini, Viviana Consonni, Raimund Mannhold, Hugo Kubinyi & Hendrik Timmerman, 2008, "Entry: Electronic substituent constants (Hammet substituent constants, σ electronic constants)," in Handbook of Molecular Descriptors, Vol. 11 of Methods and Principles in Medicinal Chemistry (book series), pp. 144–157, New York, NY, US: John Wiley & Sons, , see [https://books.google.com/books?isbn=3527613110], accessed 22 June 2015.
  * N. Chapman, 2012, Correlation Analysis in Chemistry: Recent Advances, New York, NY, US: Springer Science & Business, , see [https://books.google.com/books?isbn=1461588316], accessed 22 June 2015.