“…For mathematical results about existence, nonexistence, uniqueness, nonuniqueness and asymptotic behavior, see [2], [5], [6] and [28] for γ = 0, and [9], [12], [15], [23] and [24] for the general case. Numerical investigations can be found in [2], [7], [16], [18], [28], [30] and [38].…”