What do you understand by dependency preservation?

What do you understand by dependency preservation?
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Answer :

Given a relation R and a set of FDs F, dependency preservation states that the closure of the  union of the projection of F on each decomposed relation Ri is equal to the closure of F. i.e., ((PR1(F)) U … U (PRn(F)))+ = F+
if decomposition is not dependency preserving, then some dependency is lost in the  decomposition
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