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              Abstract
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 We give criteria for subcategories of a compactly generated algebraic triangulated
 category to be precovering or preenveloping. These criteria are formulated in terms of
 closure conditions involving products, coproducts, directed homotopy colimits and
 further conditions involving the notion of purity. In particular, we provide sufficient
 closure conditions for a subcategory of a compactly generated algebraic triangulated
 category to be a torsion class. Finally we explore applications of the previous results
 to the theory of recollements.
  
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              Keywords
              
                precover, preenvelope, definable subcategory, torsion pair,
                t-structure, recollement
               
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              Mathematical Subject Classification 2010
              
                Primary: 18C35, 18E30, 18E35, 18E40
               
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              Milestones
              
                Received: 30 April 2019
               
              
                Revised: 16 December 2019
               
              
                Accepted: 9 January 2020
               
              
                Published: 13 July 2020
               
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