The publisher apologises that upon publication of this article an error within the PDF was found. The mathematical symbols for the Q-circ time derivatives (Q with a circle on top) were not correctly typeset. Instead of rendering the symbol correctly as
$\kern2pt \mathring{\kern-2pt {\unicode{x1D64C}\kern1pt}}$ the PDF instead typeset the symbol as
${\unicode{x1D64C}}$, i.e. without the circle on top.
The online HTML version, however, was correctly typeset. The associated PDF version has been updated with corrected, consistent typesetting.