  Some important problems (e.g., in optimal transport and optimal control) have
a relaxed (or weak) formulation in a space of appropriate measures whichis much
easier to solve. However, an optimal solution $\mu$ of the latter solves the
former if and only if the measure $\mu$ is supported on a "trajectory"
$\{(t,x(t))\colon t\in [0,T]\}$ for some measurable function $x(t)$. We provide
necessary and sufficient conditions on moments $(\gamma\_{ij})$ of a measure
$d\mu(x,t)$ on $[0,1]^2$ to ensure that $\mu$ is supported on a trajectory
$\{(t,x(t))\colon t\in [0,1]\}$. Those conditions are stated in terms of
Legendre-Fourier coefficients ${\mathbf f}\_j=({\mathbf f}\_j(i))$ associated
with some functions $f\_j\colon [0,1]\to {\mathbb R}$, $j=1,\ldots$, where each
${\mathbf f}\_j$ is obtained from the moments $\gamma\_{ji}$, $i=0,1,\ldots$,
of $\mu$.
