Emergence refers to the existence or formation of collective behaviors in complex systems. Here, we develop a theoretical framework based on the eigen microstate theory to analyze the emerging phenomena and dynamic evolution of complex system. In this framework, the statistical ensemble composed of M microstates of a complex system with N agents is defined by the normalized N × M matrix
A
, whose columns represent microstates and order of row is consist with the time. The ensemble matrix
A
can be decomposed as
A
=
∑
I
=
1
r
σ
I
U
I
⨂
V
I
, where
r
=
min
(
N
,
M
)
, eigenvalue σ
I
behaves as the probability amplitude of the eigen microstate
U
I
so that
∑
I
=
1
r
σ
I
2
=
1
and
U
I
evolves following
V
I
. In a disorder complex system, there is no dominant eigenvalue and eigen microstate. When a probability amplitude σ
I
becomes finite in the thermodynamic limit, there is a condensation of the eigen microstate
U
I
in analogy to the Bose–Einstein condensation of Bose gases. This indicates the emergence of
U
I
and a phase transition in complex system. Our framework has been applied successfully to equilibrium three-dimensional Ising model, climate system and stock markets. We anticipate that our eigen microstate method can be used to study non-equilibrium complex systems with unknown order-parameters, such as phase transitions of collective motion and tipping points in climate systems and ecosystems.
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