As King (2009, p. 131) notes, this non-standard definition of the real and imaginary part of the kth complex coefficient of a power series is purposefully introduced in order to hidden ("suppress") the functional dependence of φ and ψ on r. King, Frederick W. (2009), Hilbert transforms. Volume 1, Encyclopedia of Mathematics and its Applications, vol. 124, Cambridge: Cambridge University Press, pp. xxxviii+858, ISBN978-0-521-88762-5, MR2542214, Zbl1188.44005.
As King (2009, p. 131) notes, this non-standard definition of the real and imaginary part of the kth complex coefficient of a power series is purposefully introduced in order to hidden ("suppress") the functional dependence of φ and ψ on r. King, Frederick W. (2009), Hilbert transforms. Volume 1, Encyclopedia of Mathematics and its Applications, vol. 124, Cambridge: Cambridge University Press, pp. xxxviii+858, ISBN978-0-521-88762-5, MR2542214, Zbl1188.44005.