  Let $\mathcal{W}_N$ be a quantized Borel subalgebra of $U_q(sl(2,\mc))$,
specialized at a primitive root of unity $\omega = \exp(2i\pi/N)$ of odd order
$N >1$. One shows that the $6j$-symbols of cyclic representations of
$\mathcal{W}_N$ are representations of the canonical element of a certain
extension of the Heisenberg double of $\mathcal{W}_N$. This canonical element
is a twisted $q$-dilogarithm. In particular, one gives explicit formulas for
these $6j$-symbols, and one constructs partial symmetrizations of them, the
c-$6j$-symboles. The latters are at the basis of the construction of the
quantum hyperbolic invariants of 3-manifolds.
