Synchronization in Ensembles of Oscillators: Theory of...
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Synchronization in Ensembles ofOscillators: Theory of Collective
Dynamics
A. Pikovsky
Deapartment of Physics and Astronomy, Potsdam University
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Content
• Synchronization in ensembles of coupled oscillators
• Watanabe-Strogatz theory, its relation to Ott-Antonsen eq uations
and its generalization for hierarchical populations
• Partial synchronization due to nonlinear coupling
• Self-organizing chimera
• Populations with resonant and nonresonant coupling
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Ensembles of globally (all-to-all) couples oscillators
• Physics: arrays of Josephson junctions,
multimode lasers,. . .
• Biology and neuroscience: cardiac
pacemaker cells, population of fireflies,
neuronal ensembles. . .
• Social behavior: applause in a large au-
dience, pedestrians on a bridge,. . .
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Main effect: Synchronization
Mutual coupling adjusts phases of indvidual systems, which start to keep
pace with each other
Synchronization can be treated as a nonequilibrium
phase transition!
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Attempt of a general formulation
~xk = ~f (~xk,~X,~Y) individual oscillators (microscopic)
~X =1N ∑
k~g(~xk) mean fields (generalizations possible)
~Y =~h(~X,~Y) macroscopic global variables
Typical setup for a synchronization problem:
~xk(t) – periodic orchaotic oscillators
~X(t),~Y(t) periodic or chaotic ⇒ collective synchronous rhythm
~X(t),~Y(t) stationary ⇒ desynchronization
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Description in terms of macroscopic variables
The goal is to describe the ensemble in terms of macroscopic variables
~W, which characterize the distribution of~xk,
~W =~q(~W,~Y) generalized mean fields
~Y =~h(~X(~W),~Y) global variables
as a possibly low-dimensional dynamical system
Below: how this program works for phase oscillators by virtue of Watanabe-
Strogatz and Ott-Antonsen approaches
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Kuramoto model: coupled phase oscillators
Phase oscillators (ϕk ∼ xk) with all-to-all pair-wise coupling
ϕk = ωk+ ε1N
N
∑j=1
sin(ϕ j −ϕk)
= ε
1N
N
∑j=1
sinϕ j
cosϕk− ε
1N
N
∑j=1
cosϕ j
sinϕk
= ωk+ εR(t)sin(Θ(t)−ϕk) = ωk+ εIm(Ze−iϕk)
System can be written as a mean-field coupling with the mean field
(complex order parameter Z ∼ X )
Z = ReiΘ =1N∑
keiϕk
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Synchronisation transitionεc ∼ width of distribution of frequecies g(ω)∼ “temperature”
Z
small ε: no synchronization,
phases are distributed uniformly,
mean field vanishes Z = 0
large ε: synchronization, distri-
bution of phases is non-uniform,
finite mean field Z 6= 07
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Watanabe-Strogatz (WS) ansatz
[S. Watanabe and S. H. Strogatz, PRL 70 (2391) 1993; Physica D 74 (197) 1994]
Ensemble of identical oscillators driven by the same complex field H(t)
dϕkdt
= ω(t)+ Im(
He−iϕk)
k= 1, . . . ,N
Mobius transformation to WS variables
ρ(t), Φ(t), Ψ(t), ψ1 = const, . . . ψN = const
eiϕk = eiΦ(t) ρ(t)+ei(ψk−Ψ(t))
ρ(t)ei(ψk−Ψ(t))+1
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yields WS equations
dρdt
=1−ρ2
2Re(He−iΦ) ,
dΦdt
= ω+1+ρ2
2ρIm(He−iΦ) ,
dΨdt
=1−ρ2
2ρIm(He−iΦ) .
or in a complex form for z= ρeiΦ, α = Φ−Ψ
dzdt
= iωz+12(H −z2H∗)
dαdt
= ω+ Im(z∗H)
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Why Mobius?
Phase equation ϕ = ω(t)− i(He−iϕ−H∗eiϕ)
can be rewritten for z= eiϕ as z= iωz+H −H∗z2
This Ricatti equations, for constant coefficients, has as solutions rational
functions z(t) = Az(0)+BCz(0)+D
Combination of rational functions is rational
Even for non-constant coefficients the solution can be represented as a
rational function with time-dependent parameters
cf.: Bicycle rear wheel governed by arbitrary trajectory of the front
wheel, by M. Levi
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Interpretation of WS variables
ρ measures the width of the bunch:
ρ = 0 if the mean field Z = ∑keiϕk van-
ishes
ρ = 1 if the oscillators are
fully synchronized and |Z|= 1
Φ is the phase of the bunch
Ψ measures positions of individual oscilla-
tors with respect to the bunch
ρ
Φ
Ψ
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Synchronization of uncoupled oscillators by external
forces
Ensemble of identical oscillators driven by the same complex field H(t)
dϕkdt
= ω(t)+ Im(
H(t)e−iϕk)
k= 1, . . . ,N
What happens to the WS variable ρ?
ρ → 1: synchronization
ρ → 0: desynchronization
Two basic examples oscillators and Jusephson junctions:
ϕk = ω−σξ(t)sinϕkh
2eRdϕkdt
+ Icsinϕk = I(t)
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Hamiltonian reduction
ρ =1−ρ2
2Re(H(t)e−iΦ) ,
Φ = Ω(t)+1+ρ2
2ρIm(H(t)e−iΦ) .
in variables
q=ρcosΦ√
1−ρ2, p=−
ρsinΦ√
1−ρ2,
reduces to a Hamiltonian system with Hamiltonian,
H (J,Φ, t) = Ω(t)J−H(t)
√
2J(2J+1)2
sinΦ .
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Action-angle variables
J =ρ2
2(1−ρ2), Φ
Hamiltonian reads
H (J,Φ, t) = Ω(t)J−H(t)
√
2J(2J+1)2
sinΦ
Synchrony: H ,J → ∞Asynchrony: H ,J → 0
For general noise: “Energy” grows ⇒ synchronization by common noise
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Analytic solution for the initial stage
Close to asynchrony: energy is small, equations can be linearized ⇒
exact solution
H ,J ∼ σ2t
Close to synchrony:
H ,J ∼ exp[λt]
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10-2
100
102
104
106
0.1 1 10 100 1000 10000
t
〈J〉
σ = 0.05
σ = 0.15
σ = 0.5
10-2
100
102
104
106
0.0001 0.001 0.01 0.1 1 10
σ2t/8
〈J〉
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Globally coupled ensembles
Kuramoto model with equal frequencies
ϕk = ω+ εIm(Ze−iϕk)
belongs to the WS-class
dϕkdt
= ω(t)+ Im(
H(t)e−iϕk)
k= 1, . . . ,N
where H is the order parameter
Z = ReiΘ =1N∑
keiϕk
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Complex order parameters via WS variables
Complex order parameter can be represented via WS variables as
Z = ∑k
eiϕk = ρeiΦγ(ρ,Ψ) γ = 1+(1−ρ−2)∞∑l=2
Cl(−ρe−iΨ)l
where Cl = N−1∑keil ψk are Fourier harmonics of the distribution of
constants ψk
Important simplifying case (adopted below):
Uniform distribution of constants ψk
Cl = 0 ⇒ γ = 1 ⇒ Z = ρeiΦ = z
In this case WS variables yield the order parameter directly !18
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Closed equation for the order parameter for the
Kuramoto-Sakaguchi model
Individual oscillators:
ϕk = ω+ ε1N
N
∑j=1
sin(ϕ j −ϕk+β) = ω+ εIm(Zeiβe−iϕk)
Equation for the order parameter is just the WS equation:
dZdt
= iωZ+ε2eiβZ−
ε2e−iβ|Z|2Z
Closed equation for the real order parameter R= |Z|:
dRdt
=ε2R(1−R2)cosβ
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Simple dynamics in the Kuramoto-Sakaguchi model
dRdt
=ε2R(1−R2)cosβ
Attraction: −π2 < β < π
2 =⇒
Synchronization, all phases identical ϕ1 = . . . = ϕN, order parameter
large R= 1
Repulsion: −π < β <−π2 and π
2 < β < π =⇒
Asynchrony, phases distributed uniformely, order parameter vanishes
R= 0
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Linear vs nonlinear coupling I
• Synchronization of a periodic autonomous oscillator is a nonlinear
phenomenon
• it occurs already for infinitely small forcing
• because the unperturbed system is singular (zero Lyapunov expo-
nent)
In the Kuramoto model “linearity” with respect to forcing is assumed
x = F(x)+ ε1f1(t)+ ε2f2(t)+ · · ·
ϕ = ω+ ε1q1(ϕ, t)+ ε2q2(ϕ, t)+ · · ·
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Linear vs nonlinear coupling II
Strong forcing leads to “nonlinear” dependence on the forcing amplitude
x = F(x)+ εf(t)
ϕ = ω+ εq(1)(ϕ, t)+ ε2q(2)(ϕ, t)+ · · ·
Nonlineraity of forcing manifests itself in the deformation/skeweness of
the Arnold tongue and in the amplitude depnedence of the phase shift
forcing frequencyforc
ing
ampl
itude
ε linear nonlinear
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Linear vs nonlinear coupling III
Small each-to-each coupling ⇐⇒ coupling via linear mean field
1 N2 3
ΣX
Y
linearunit
Strong each-to-each coupling ⇐⇒ coupling via nonlinear mean field
[cf. Popovych, Hauptmann, Tass, Phys. Rev. Lett. 2005]
1 N2 3
ΣX
Y
nonlinearunit
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Nonlinear coupling: a minimal model
We take the standard Kuramoto-Sakaguchi model
ϕk = ω+ Im(He−iϕk) H ∼ εe−iβZ Z=1N ∑
jeiϕ j = ReiΘ
and assume dependence of the acting force H on the “amplitude” of the
mean field R:
ϕk = ω+A(εR)εRsin(Θ−ϕk+β(εR))
E.g. attraction for small Rvs repulsion for large R
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WS equations for the nonlinearly coupled ensemble
dRdt
=12R(1−R2)εA(εR)cosβ(εR)
dΦdt
= ω+12(1+R2)εA(εR)sinβ(εR)
dΨdt
=12(1−R2)εA(εR)sinβ(εR)
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Full vs partial synchrony
All regimes follow from the equation for the order parameter
dRdt
=12R(1−R2)εA(εR)cosβ(εR)
Fully synchronous state: R= 1, Φ = ω+ εA(ε)sinβ(ε)Asynchronous state: R= 0Partially synchronous bunch state
0< R< 1 from the condition A(εR) = 0:
No rotations, frequency of the mean field = frequency of the oscillations
Partially synchronized quasiperiodic state
0< R< 1 from the condition cosβ(εR) = 0:
Frequency of the mean field Ω = ϕ = ω±A(εR)(1+R2)/2Frequency of oscillators ωosc= ω±A(εR)R2
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Self-organized quasiperiodicity
• frequencies Ω and ωoscdepend on ε in a smooth way
=⇒ generally we observe a quasiperiodicity
• attraction for small mean field vs repulsion for large mean field
=⇒ ensemble is always at the stabilty border β(εR) =±π/2, i.e.
in a
self-organized critical state
• critical coupling for the transition from full to partial synchrony:
β(εq) =±π/2• transition at “zero temperature” like quantum phase transition
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Simulation: loss of synchrony with increase of coupling
0 0.2 0.4 0.6 0.8 10
0.5
1
1
1.1
coupling ε
orde
rpa
ram
eter
Rω
osc,
Ωm
f
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Hierarchically organized populations of oscillators
We consider populations consisting of M identical subgroups (of different
sizes)
a b c d
Each subgroup is described by WS equations
⇒ system of 3M equations completely describes the ensemble
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dρa
dt=
1−ρ2a
2Re(Hae−iΦa) ,
dΦa
dt= ωa+
1+ρ2a
2ρaIm(Hae−iΦa) ,
dΨa
dt=
1−ρ2a
2ρaIm(Hae−iΦa) .
General force acting on subgroup a:
Ha =M
∑b=1
nbEa,bZb+Fext,a(t)
nb: relative subgroup size
Ea,b: coupling between subgroups a and b30
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Thermodynamic limit
If the number of subgroups M is very large, one can consider a as a
continuous parameter and get a system
∂ρ(a, t)∂t
=1−ρ2
2Re(H(a, t)e−iΦ)
∂Φ(a, t)∂t
= ω(a)+1+ρ2
2ρIm(H(a, t)e−iΦ)
∂Ψ(a, t)∂t
=1−ρ2
2ρIm(H(a, t)e−iΦ)
H(a, t) = Fext(a, t)+∫
dbE(a,b)n(b)Z(b)
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Relation to Ott-Antonsen equations[E. Ott and T. M. Antonsen, CHAOS 18 (037113) 2008]
• in the case when subgroups differ only by frequency
• the coupling is global but nonlinear E(ω,ω′) = εA(εR)eiβ(εR)
• for a particular case when the complex order parameter for each sub-
group is expressed via the WS variables as Z(ω) = ρ(ω)eiΦ(ω)
we obtain Ott-Antonsen integral equations
∂Z(ω, t)∂t
= iωZ+12H −
Z2
2H∗
H = εA(εR)eiβ(εR)Y Y= ReiΦ =∫
dωn(ω)Z(ω)32
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OA equations for Lorentzian distribution of frequencies
If
n(ω) =∆
π((ω−ω0)2+∆2)
then the integral Y =∫
dωn(ω)Z(ω) can be calculated via residues as
Y = Z(ω0+ i∆)
This yields an ordinary differential equation for the order parameter Y
dYdt
= (iω0−∆)Y+12
εA(εR)(eiβ(εR)−e−iβ(εR)|Y|2)Y
Hopf normal form / Landau-Stuart equation/ Poincare oscillator
dYdt
= (a+ ib− (c+ id)|Y|2)Y
33
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Nonidentical oscillators with nonlinear coupling
Lorentzian distribution of natural frequencies n(ω)⇒ standard “finite temperature” Kuramoto model of globally coupled
oscillators with nonlinear coupling
(attraction for a small force, repulsion for a large force)
Novel effect: Multistability
Different partially synchronized states coexist for the same parameter
range
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Multistability of synchronous and asynchronous states
in a Kuramoto model with nonlinear coupling
Nonlinear phase shift:
β = β0+ ε2R2
as: asynchronous
s: (partially) synchronous
ns: n coexisting synchronous states
-2 0 2
2
3
4
5
asas
as/s
as/2s3s
s
as/s
2s
as/2s
as/3s
β0
ε
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0
0.2
0.4
0.6
0.8
2 2.5 3 3.5 4 4.5 5
00.5
11.5
22.5
3
Ω
ε
R
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Experiment[Temirbayev et al, PRE, 2013]
Linear coupling Nonlinear coupling
0.2
0.6
1
0
1
2
0 0.2 0.4 0.6 0.8 1
1.10
1.12
1.14
1.16
(a)
(b)
(c)
ε
RA
min
[V]
fm
f,f
i[k
Hz]
0.2
0.6
1
0
1
2
0 0.2 0.4 0.6 0.8 1
1.10
1.12
1.14
1.16
ε
(d)
(e)
(f)
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Chimera states
Y. Kuramoto and D. Battogtokh observed in 2002 a symmetry breaking
in non-locally coupled oscillators H(x) =∫
dx′ exp[x′−x]Z(x′)
This regime was called “chimera” by Abrams and Strogatz
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Chimera in two subpopulations
Model by Abrams et al:
ϕak = ω+µ
1N
N
∑j=1
sin(ϕaj −ϕa
k+α)+(1−µ)N
∑j=1
sin(ϕbj −ϕa
k+α)
ϕbk = ω+µ
1N
N
∑j=1
sin(ϕbj −ϕb
k+α)+(1−µ)N
∑j=1
sin(ϕaj −ϕb
k+α)
Two coupled sets of WS equations: ρa = 1 and ρb(t) quasiperiodic are
observed
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Chimera in experiments I
Tinsley et al: two populations of chemical oscillators
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Chimera in experiments IIer bei Wikipedia nach „Syn-
sich mit einer großen Vielfalt an Er-
klärungen konfrontiert: Angesichts
der breit gefächerten Verwendung
in Film und Fernsehen, Informatik,
Elektronik usw. geht die Bedeutung
Eri
k A
. Ma
rte
ns,
M
PI
für
Dy
na
mik
un
d S
elb
sto
rga
nis
ati
on
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Self-emerging and turbulent chimera states at nonlinear
coupling (with G. Bordyugov)
A one-dimensional array of oscillators with non-local coupling
∂A∂t
= A−|A|2+ εeiβ0+iβ1|B|2B
B(x, t) =∫ l
−ldx′ G(x−x′)A(x′, t)
β1 = 0 corresponds to linear coupling where chimera states (part of
oscillator synchronous, part quasiperiodic) coexist with a stable syn-
chronous state [Y. Kuramoto and D. Battogtokh, 2002].
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Transition to static and turbulent chimera
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Multifrequency populations
ω1
ω2
ω3
ω4
Each subpopulation is described by WS/OA equations for an effective
”oscillator”
Equivalent to many coupled oscillators
Amplitude = order parameter in the subpopulation
Phases = collective phases of mean fields44
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Multifrequency I: Non-resonantly interacting ensembles
[M. Komarov, A. P., Phys. Rev. E, v. 84, 016210 (2011)] Frequencies are different
– all interactions are non-resonant
Only amplitudes of the order parameters can be involved in the coupling
between subpopulations
General equations are of type
ρl = (−∆l −Γlmρ2m)ρl +(al +Almρ2
m)(1−ρ2l )ρl , l = 1, . . . ,L
where Γlm and Alm decsribe the coupling
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Competition for synchrony
(a)
00.5
1
0
0.5
1
0
0.5
1
C1
C2
C3
(b)
00.5
1
0
0.5
1
0
0.5
1
C1
C2
C3
C12
C23
C13
Only one ensemble is synchronous – depending on initial conditions46
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Heteroclinic synchrony cycles
Sequential synchrony (partial or full) in populations
(a) (b)
(c)0 500 1000 1500 2000 2500 3000
0
0.2
0.4
0.6
0.8
1
time
ρ 1,2,
3
ρ1
ρ2
ρ3
(d)0 2000 4000 6000 8000 10000
0
0.2
0.4
0.6
0.8
1
time
ρ 1,2,
3
ρ1
ρ2
ρ3
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Chaotic synchrony cycles
Order parameters demonstrate chaotic oscillations
0
1
0 2000 4000 6000 8000 100000
1
time
υ1
ρ1
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Multifrequency II: Resonantly interacting ensembles
[M. Komarov and A. P., Phys. Rev. Lett. 110, 134101 (2013)]
ω1
ω2
ω3
Most elementary nontrivial resonance ω1+ω2 = ω349
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On the level of individual oscillators (phase φ from ω1, phase ψ from
ω2, phase θ from ω3 = ω2+ω1) one has to take into account triple
interactions :
φk = . . .+Γ1∑m,l sin(θm−ψl −φk+β1)
ψk = . . .+Γ2∑m,l sin(θm−φl −ψk+β2)
θk = . . .+Γ3∑m,l sin(φm+ψl −θk+β3)
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Set of three OA equations
On the level of effective oscillators describing order parameters, one has
a triplet of Stuart-Landau equations with resonant coupling terms
z1 = z1(iω1−δ1)+(ε1z1+ γ1z∗2z3−z21(ε
∗1z∗1+ γ∗1z2z∗3))
z2 = z2(iω2−δ2)+(ε2z2+ γ2z∗1z3−z22(ε
∗2z∗2+ γ∗2z1z∗3))
z3 = z3(iω3−δ3)+(ε3z3+ γ3z1z2−z23(ε
∗3z∗3+ γ∗3z∗1z∗2))
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Regions of synchronizing and desynchronizing effect
from triple coupling
−π
0
π
−π 0 π
+++ ++-++-
++-
++-
-++
-++
+-+
+-+
β1−β2
2β3+
β 1+
β 2
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Bifurcations in dependence on phase constants
Different transitions from full to partial to oscillating synchrony
2.4 2.6 2.8 3.00.0
0.2
0.4
0.6
0.8
1.0
ρ3Limit cycle amplitudeStable fixed pointUnstable fixed point
A-H
TC
πβ3 +β1
2.0 2.2 2.4 2.6 2.8 3.00.0
0.2
0.4
0.6
0.8
1.0
ρ3
Limit cycle amplitudeStable fixed pointUnstable fixed pointS-N
A-H
TC
β3 +β1π
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Chaos of order parameters
0.3
0.6 0 0.3 0.6
0
0.3
0.6
ρ1
ρ2
ρ3ρ3
0.6 0.8 1.0 1.2 1.4
β1
−0.05
0.00
0.05
0.10
0.15
0.20
0.25
Λ
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Conclusions
• Closed system for order parameters evolution (WS variables for iden-
tical, OA for non-identical)
• Nonlinear coupling I: “nonequilibrium quantum phase transition” from
full to partial synchrony
• Nonlinear coupling II: self-organized chimera
• Multifrequency populations can be described in terms of order pa-
rameters as ”coupled oscillators”
• Resonantly and nonresonantly interacting populations – quasiperi-
odic partial synchrony, competition for synchrony and synchroniza-
tion death, heteroclinic synchrony cycles, chaotic synchrony dinam-
ics
55