more on complexity measures
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More on complexity measures. Statistical complexity. J. P. Crutchfield. The calculi of emergence. Physica D . 1994. complex random. C. randomness, H. I. H. Entropy and algorithmic complexity associate maximum complexity with randomness - PowerPoint PPT PresentationTRANSCRIPT
More on complexity measures
Statistical complexityJ. P. Crutchfield. The calculi of emergence. Physica D. 1994
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complex random
• Entropy and algorithmic complexity associate maximum complexity with randomness pure order and pure noise are not “complex” complex systems have
intricate structure on multiple scales repeating patterns continual variation, …
complexity lies between order and chaos Wolfram’s class 4 CAs Langton’s “edge of chaos”
• Mutual information shows complexity RBN transition example (also k-SAT):
are there other measures like this?
randomness, H
C
H
I
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when randomness = noise
• the measures so far assume that randomness is information even logical depth
Randomness is not very “deep” information
• Sometimes, the “randomness” actually is information the output of good compression algorithms is highly
“random” else the remaining structure could be used to compress it more
“any sufficiently advanced communication is indistinguishable from noise”
crypto functions output “random” strings else the remaining structure could be used to break the code
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randomness and noise
• in the real world, (some) randomness is just “noise” of no interest, carrying no “information”
• these pictures are all different microscopically, but all just “white noise” macroscopically
the differences are not important information measures “overfit” noise as data
this kind of noisy randomness is intuitively simple a small change to the noise, is just the same noise
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to model a coin toss …
• how would you create an ensemble of random bit strings?
… just toss a coin!
in other words, use a stochastic automaton
• that’s quite a short description conforming to our intuition that random strings are not
very complex
H | ½ T | ½
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Statistical complexity
• In certain circumstances, we can use theory of discrete computation and statistics to create equivalent models Needs a discrete stochastic process that is conditionally stable
Future states do not depend on time, but only on previous states
• Complexity, C is the size of a minimal model yielding a finite description that is at the least computationally powerful level infer the machine from data ensemble
The collection of observed strings generated by process of interest
• Statistical complexity ignores the “computational resource” So randomness and periodicity have zero complexity
J. P. Crutchfield. The calculi of emergence. Physica D. 1994
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• the inferred minimal model is called an - machine minimal model
size of the minimal stochastic machine
finite description size of machine does not grow unboundedly with the size of the
state
least computationally powerful level e.g. finite state automaton, stack machine, UTM
• Intuition: Each observation represents a state, which incorporates an
indirect indication of the hidden environment States that lead to the same next state help to predict the
environment Causal states
An – machine captures a minimal sequence of causal statesJ. P. Crutchfield. The calculi of emergence. Physica D. 1994
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Consider a simple process
• The process is a simple automaton
• A system with a two-symbol alphabet, α = {0,1} Two recurrent states, A and B
• State A can, with equal probability, emit a 0 and return to itself emit a 1 and go to state B
• State B always emits 1 and goes to A
• But all we have is a black-box process
C. R. Shalizi, K. L. Shalizi, J. P. Crutchfield. An algorithm for pattern discovery in time series. 2002. http://arxiv.org/pdf/cs/0210025v3.pdf
This is Weiss’s “even process”: •a 1 cannot be completely surrounded by other 1s
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Record the process output
• We need to deduce the automaton from data observations
• Run the process many times To get statistically useful data
e.g. 104 runs to word length = 4
C. R. Shalizi, K. L. Shalizi, J. P. Crutchfield. An algorithm for pattern discovery in time series. 2002. http://arxiv.org/pdf/cs/0210025v3.pdf
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example : “even” process (1)
• Work out probabilities and infer a machine “homogonisation” because homogeneous states are
merged Merge is the main source of error – need a lot of
observations
For full calculation, see:
C. R. Shalizi, K. L. Shalizi, J. P. Crutchfield. An algorithm for pattern discovery in time series. 2002. http://arxiv.org/pdf/cs/0210025v3.pdf
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example : “even” process (2)
• Check all states have incoming transitions Reachability
• Remove transient states A and B form a transient cycle
The only exit is to produce a 0 and go to C
Every C state goes to C (adding 0) or D (adding 1) Every state in D goes to C (adding 1)
“Determinisation”
• Final – machine has states C and D only
C. R. Shalizi, K. L. Shalizi, J. P. Crutchfield. An algorithm for pattern discovery in time series. 2002. http://arxiv.org/pdf/cs/0210025v3.pdf
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– machines and stability
• Replicating a process in an – machine requires stability Previous states aren’t always “causal” in unstable systems
• Stability is related to temporal scale Recall flocking
At the level of birds, apparently arbitrary motion, few patterns At the level of the flock, coherent, apparently co-ordinated motion
• So, can change level (scale) to one where there is stability A bit like choosing the level to represent in differential equations
• We can tell a system is not suitably stable if the inferred – machine changes with the word length That is, as the process runs over time, the – machine has to
change to express its statistical behaviour
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– machines and continuous systems
• Most natural systems are continuous
• Symbolic dynamics used to extract discrete time systems Partition the state space and label each partition with a
symbol Over time, each point in the state space has a sequence of
symbols Its symbol at each observation point in its past and future Loses information Often deterministic continuous system gives stochastic
discrete system
http://vserver1.cscs.lsa.umich.edu/~crshalizi/notabene/symbolic-dynamics.html (and citations)
Ґ
ЖЦ
ЂϠ
a
Ґ
ЖЦ
ЂϠ
aPoint a is in region Ж at time t
Over a series of discrete time observations, a moves through different regions: ... Ж Ж Ж ϠϠЂЂЂЂ Ж Ж …
aaa
aa
aa
a
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Symbolic dynamics (1)
• recast a continuous (space / time) dynamical system into a discrete one
• partition the continuous phase space U into a finite number of sets, each labelled with a unique element from a finite alphabet : Ui
• observe the system at discretised time intervals, and note the label of the set Ui it
occupies, to give a sequence of symbols: d c a a b d d a a … rationale : sequences represent “results” of
“measurements” of the underlying system
a b
c
d
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Symbolic dynamics (2)
• the symbolic dynamics of the system is the set of all sequences that can be produced (different initial conditions, etc)
defines a language
• analyse the dynamics of these sequences using entropy, mutual information, -
machines, etc.
• e.g. Crutchfield’s analysis of the complexity and entropy of the logistic map: see J. P. Crutchfield. The calculi of emergence. Physica D. 1994
a b
c
d
3.5 < < 4
= 3.5699…
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Analysis results for logistic map
• periodic behaviour, small H, small C automaton size = the period
• chaotic behaviour, large H, small C a small automaton captures the random
behaviour (“coin toss”) randomness, H
C
J. P. Crutchfield. The calculi of emergence. Physica D. 1994
• Complex behaviour, mid H, large C near the transition from periodic to chaotic behaviour
(“edge of chaos”) there is structure “on all scales”
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Another complexity measure:multi-information
• recall mutual information between two systems:
where H(X) is the entropy of system X H(X,Y) is the joint entropy of the systems X and Y
I = 0 if X and Y are independent
• For subsystems X1, X2, and the overall system
X1,2 ,this gives:
( ; ) ( ) ( ) ( , )I X Y H X H Y H X Y
1,2 1 2 1,2( ) ( ) ( ) ( )I X H X H X H X
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multi-information (1)
• multi-information generalises this to n subsystems of an overall system
• System X= X1,2,…n
• Subsystems X1, X2, …, Xn
where
• MI = 0 if all the subsystems are independent
1,2 1 2 1,2( ) ( ) ( ) ( )MI X H X H X H X
( ) ( ) ( )ii
MI X H X H X
2 1( ) log ( , , )nH X p x x
M. Studeny, J. Vejnarova. The multiinfomration function as a tool for measuring stochastic dependence. In Learning in Graphical Models. Kluwer, 1998
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multi-information (2)
• now consider partitioning the top level system X into
two subcomponents Xa, comprising subsystems X1, …,
Xk , and Xb comprising subsystems Xk+1, …, Xn
• the relationship between the multi-information of the whole system and its two big components is
rearranging, and substituting
• so (unless the subsystems Xa and Xb
are independent) : the MI of the whole is bigger than the parts
1 1
( ) ( ) ( ); ( ) ( ) ( )k n
a i a b i bi i k
MI X H X H X MI X H X H X
X1 X2 … Xk … Xn
Xa Xb
X
( ) ( ) ( )a bMI X MI X MI X
( ) ( ) ( ) ( ; )a b a bMI X MI X MI X I X X
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multi-information (3)
• instead of considering one big subcomponent comprising k subsystems, now consider all possible such big subcomponents of k subsystems, each comprising subsystems Xi1,
…, Xik
• consider the average multi-information of these, note that and
• given the MI of the whole is bigger than the parts, we have
• so the MI increases with the size of the subsystems considered
( )kMI X
( ) ( )nMI X MI X 1( ) 0MI X
1( ) ( ) ( ) 0k kMI X MI X MI X
X1 X2 X3
X
X1 X2
X1 X3
X2 X3
X12
X22
X32
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multi-information = complexity
• complexity is the difference between actual increase of this average, and a linear increase:
• C 0
• C is low if the system is random all subsystems are independent, and so MI = 0
• C is low if the system is homogeneously structured average MI increases linearly
• C is high in the intermediate case, inhomogeneous groupings and clumpings
high, non-linearly increasing, average MIs
1
( ) ( ) ( )n
kkMI n
k
C X MI X MI X
G. Tononi, et al. A measure for brain complexity: relating functional segregation and integration in the nervous system. PNAS 91:5033-37, 1994
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which complexity measure?
• unconditional entropy is probably not appropriate counts randomness as maximally “complex” entropy variance readily calculated
between different space / time parts of self
• algorithmic complexity K useful for theoretical analyses, but not for analysing practical
results
• conditional entropy/mutual information/multi-information between two systems
which can be between different space / time parts of self appears to be maximised around interesting transitions
or between hierarchical levels of a system
• statistical complexity C of single system; appears to be maximised at “edge of chaos”
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Some general sources
• http://www.scholarpedia.org/article/Complexity
• R. Badii, A. Politi. Complexity. Cambridge University Press. 1997
• J. P. Sethna. Statistical mechanics. Oxford University Press. 2006