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This equation arises in the study of the spread of an infectious disease that does not induce permanent immunity. For
detailed meanings of the various functions arising in (1.1), see [5] and also [1,3,4,6], for more results, see [2,7–10,13] and
the references therein. In [5], the author studied the existence of a unique bounded continuous and nonnegative solution
of (1.1) for t ∈ R+ under appropriate assumptions on A and B, and also obtained sufficient conditions for the convergence
of the solution to a limit when t → ∞.
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