A subspace 
of 
is called a retract of 
if there is a continuous map 
(called a retraction)
such that for all 
and all 
,
1. 
, and
2. 
.
Equivalently, a subspace 
of 
is called a retract of 
if there is a continuous map 
(called a retraction)
such that for all 
,
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