  The Bessel operator, that is, the Schr\"odinger operator on the half-line
with a potential proportional to $1/x^2$, is analyzed in the momentum
representation. Many features of this analysis are parallel to the approach \`a
la K. Wilson to Quantum Field Theory: one needs to impose a cutoff, add
counterterms, study the renormalization group flow with its fixed points and
limit cycles.
