In 1963 a partial differential equation with a convolution nonlinearity was introduced
in connection with a quantum mechanical many-body problem, namely the gas of
bosonic particles. This equation is mathematically interesting for several reasons.
Although the equation was expected to be valid only for small values of the
parameters, further investigation showed that predictions based on the equation
agree well over the
entire range of parameters with what is expected to be true
for the solution of the true many-body problem. Additionally, the novel
nonlinearity is easy to state but seems to have almost no literature up to now.
Finally, the earlier work did not prove existence and uniqueness of a solution,
which we provide here along with properties of the solution such as decay at
infinity.