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Projection algebras are $M$-sets for the monoid
$M=(N^\infty, min)$ which are used by computer scientists for
algebraic specification of process algebras. In contrast to the
case of modules it is well known that the Baer Criterion does not
generally hold for injectivity of $M$-sets for an arbitrary monoid
$M$.\Here, introducing a closure operator, we prove that the Baer
Criterion does hold for injectivity of projection algebras.
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