  Various measures have been suggested recently for quantifying the coherence
of a quantum state with respect to a given basis. We first use two of these,
the l_1-norm and relative entropy measures, to investigate tradeoffs between
the coherences of mutually unbiased bases. Results include relations between
coherence, uncertainty and purity; tight general bounds restricting the
coherences of mutually unbiased bases; and an exact complementarity relation
for qubit coherences. We further define the average coherence of a quantum
state. For the l_1-norm measure this is related to a natural 'coherence radius'
for the state, and leads to a conjecture for an l_2-norm measure of coherence.
For relative entropy the average coherence is determined by the difference
between the von Neumann entropy and the quantum subentropy of the state, and
leads to upper bounds for the latter quantity. Finally, we point out that the
relative entropy of coherence is a special case of G-asymmetry, which
immediately yields several operational interpretations in contexts as diverse
as frame-alignment, quantum communication and metrology, and suggests
generalising the property of quantum coherence to arbitrary groups of physical
transformations.
