Text and Data Mining valid from 2018-03-21
Received: 28 August 2017
Accepted: 29 January 2018
First Online: 21 March 2018
Change Date: 12 June 2018
Change Type: Correction
Change Details: The original version of this Article contained an error in Equation 1. A factor of ‘c’ was included in the right-hand term. This has been corrected in the PDF and HTML versions of the Article.
: The authors declare no competing interests.