  I point out that the thermal one-point functions of a pair of relevant
operators in massive free QFTs, in odd dimensions and in the presence of an
imaginary chemical potential for a U(1) global charge, are given by certain
classes of single-valued polylogarithms. This result is verified by a direct
calculation using the thermal OPE. The complex argument of the polylogarithms
parametrize a two-dimensional subspace of relevant deformations of generalised
free CFTs, while the rank of the polylogarithms is related to the dimension d.
This may be compared with the well-known representation of single-valued
polylogarithms as multiloop Feynman amplitudes. As an example, the thermal
one-point function of the U(1) charge in d-dimensions generalises the thermal
average of the twist operator in a pair of harmonic oscillators and is given by
the well-known conformal ladder graphs in four dimensions.
