The most general stress function in three dimensions is given by the symmetric
Beltrami tensor, which consists of six scalar functions. In this brief communication,
we explore the possibility of expressing a three-dimensional state of stress by
means of a single scalar function, in the spirit of the Airy stress function in
two dimensions. The price to pay for such a severe restriction is a loss of
generality, in the sense that not every divergence-free stress field can be
derived from the single stress function. Clearly, any such single scalar function
gives rise to a particular case of the Beltrami tensor. One of these particular
cases exhibits an interesting analogy with the tensor of inertia of a rigid
body.