  We show that the modular symbol $(0,\infty)$, considered as an element of the
dual of Emerton's completed cohomology, interpolates Kato's Euler system at
classical points, and we deduce from this a factorisation of Beilinson-Kato's
system as a product of two symbols $(0,\infty)$ (an algebraic analog of
Rankin's method). The proof uses the $p$-adic local Langlands correspondence
for ${\mathbf GL}_2({\mathbf Q}_p)$ and Emerton's factorization of the
completed cohomology of the tower of modular curves for which we provide a new
proof resting upon the construction of a Kirillov model for the completed
cohomology, and which we refine by imposing conditions at classical points; the
existence of such a refinement is a manifestation of an analyticity property
for $p$-adic periods of modular forms.
