  In 1963, a Simple Approach was developed to study the ground state energy of
an interacting Bose gas. It consists in the derivation of an Equation, which is
not based on perturbation theory, and which gives the exact expansion of the
energy at low densities. This Equation is expressed directly in the
thermodynamic limit, and only involves functions of $3$ variables, rather than
$3N$. Here, we revisit this approach, and show that the Equation yields
accurate predictions for various observables for all densities. Specifically,
in addition to the ground state energy, we have shown that the Simple Approach
gives predictions for the condensate fraction, two-point correlation function,
and momentum distribution. We have carried out a variety of tests by comparing
the predictions of the Equation with Quantum Monte Carlo calculations, and have
found remarkable agreement. We thus show that the Simple Approach provides a
new theoretical tool to understand the behavior of the many-body Bose gas, not
only in the small and large density ranges, which have been studied before, but
also in the range of intermediate density, for which little is known.
