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Given a diagram of Π–algebras (graded groups equipped with an action
of the primary homotopy operations), we ask whether it can be realized
as the homotopy groups of a diagram of spaces. The answer given here
is in the form of an obstruction theory, of somewhat wider application,
formulated in terms of generalized Π–algebras. This extends a
program begun in [J Pure Appl Alg. 103 (1995) 167-188] and [Topology
43 (2004) 857-892] to study the realization of a single Π–algebra.
In particular, we explicitly analyze the simple case of a single map,
and provide a detailed example, illustrating the connections to higher
homotopy operations.
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