Logistic map

The logistic map is a one dimensional discrete-time non-linear system, that lends itself to being used as information source of known entropy (c.f. Schuster, "Introduction to non-linear dynamics"). There is a great deal in the literature describing the behaviour of the logisitic map so we shan't repeat all the details here. Just some.

The following figure shows the iterates of the logistic map as a function of the control parameter r. (See also the T-entropy views at bottom of  page) Superposed on this is a plot of the Lyapunov exponent computed at increments of .0001. The significance of the Lyapunov exponents is its characterization of the dynamics. Essentially a negative value is indicative of stable periodic solutions to the recurrence relation, whereas positive values are indicative of unpredictable. unstable dynamics. The larger the Lyapunov exponent, the more unpredictable the system. In d-dimensional phase space there will be d exponents, one for each degree of freedom.








Kolmogorov recognised (1958) that the phase space trajectory of a dynamical system could be partitioned into volume elements and by suitably labelling each element one could thus symbolically encode the trajectory. By further associating with each volume element visitation probabilities he proposed that one could in principle if not in practice compute the corresponding Shannon entropy for the system dynamics. The optimum partition, maximising the Shannon entropy fr the system is referred to as the Kolmogorov-Sinai entropy. It is well known that certain coarse grained  partitions of the logistic map state space will yield finite alphabetic encodings of the dynamics that correspond to the KS-entropy. One such "generating" bi-partition, i.e. yielding binary encodings of the dynamics, exists at p=0.5. That is if at time t the state xt * p we encode the symbol st = '1' else xt < p encode st = '0'.

Pesin proved (1977) that for certain classes of system including the one dimensional maps, like the logisitic map, henon map and tent map, the KS-entropy is equal to the sum of positive Lyapunov exponents. For one dimensional systems only a single exponent exists for each. For the Tent map the Lyapunov exponent has been computed in closed form (c.f. Schuster). For the Logistic map, standard methods can be employed to compute the exponent with good precision, from the observed time series. The above graph depicts in green the Lyapunov exponent for the logistic map (courtesy R. Steuer).

Whereas the evaluation of the Shannon entropy from a string of symbols ordinarily requires á priori knowledge of the source distribution, one may for the binary encodings of the logistic map dynamics, according to Pesin's identity use the positive Lyapunov exponent to give the entropy of the source. Thus the shannon entropy of the binary strings may be infered indirectly by way of pesin's identity.

On the other hand, Deterministic -IT allows us to also compute from the string encodings, a quantity which we refer to as the T-entropy. As illustrated in the following graph, it is clear that the T-entropy closely tracks the positve Lyapunov exponent over the whole of the range of possible entropies 0 - ln(2) (nats)  (i.e. corresponding to normalised entropies 0 - 1 bits/bit). In contrast to Shannon's entropy, the T-entropy is computed directly from a finite length sample string without reference to the source symbol probabilities. By using the logistic map to 'calibrate' this alternative measure we may apply the measure to the braoder range of applications where information measurement is desirable.

In essence we may deduce from a string the equivalent Shannon source entropy for which the string might be considered typical. Moreover, and as we shall subsequently demonstaret, we may further confirm stationarity or otherwise of the source, without resorting to statistical techniques. There are a number of important aspects to appying the measure generally which we consider here.

The graph below depicts for the logisitic map: i) the Lyapunov exponent (green) , and ii)  T-entropy values (red) computed from sample strings of 10Mbits each and encoded from the generating bi-partition described earlier. The information rate (entropy = positve Lyapunov exponent) of the logisitic map varies from 0 through to the maximum possible 1 (normalised entropy scale) (y-axis) as a function of a single control parameter r, ranging from 3.5 - 4 (x-axis). The relationship between the entropy and r is in fact  complex and illustrated here in simplified form for values of r, at intervals of .001. In the absence of a formal proof, the graph confirms implicitly that the T-entropy is measuring essentially a quantity that corresponds closely to the Shannon-entropy of the corresponding source range for which the sample string may be considered typical and it  does this across the whole .
 
 

Lyapunov exponent (green) and T-entropy (red) computed
from 10Mbit long string encodings about bi-partition (c = 0.5).
This graph computed from over 4000 10Mbit files in a matter of hours!
(Not using "tcalc" since this runs only as O(n^2), but using a new implementation "qtcalc", by Yang Jia ... runs as O(n)! )







The next graph plots the T-entropy as a function of the state space partition parameter. This essentially confirmes that the entropy
is maximal. irrespective of the value of r, for the partition equal to the critical value c = 0.5.



A shell script for generating binary encodings of the logisitc map trajectory

Copy the following into a file called "lgst" say.  Make the file executable by typing: chmod +x lgst

This simple script doesn't check the validity of your arguments!
Typing the command
./lgst 3.7 0.65 10000 > myfile
will generate a string of 10000 bits into the file myfile. The script is not fast, but has the advantage that the scale (resolution) of the computation may be set by the user. Its Shannon entropy will correspond to the positive Lyapunov exponent at 3.7.
(You may use tcalc to compute its t-entropy. To interpret the results of this see using tcalc )
#!/bin/sh
# the logistic map is a one dimensional discrete time non-linear dynamical system
# the state of the system x, at time t, is given by the recurrence relation:
#  x_{t} = r x_{t-1} (1 -x_{t-1})
RVAR=$1        # biotic potential, r, should be in range (1,4)
INITVAR=$2     # initial state should be in interval (0,1), e.g.: 0.65
LENGVAR=$3
echo "scale=30; r=$RVAR; x=$INITVAR; for(i=0;i< $LENGVAR;i++) {x = r*x*(1-x); x>.5}" | bc | tr -d "\n"



Logistic map as an information source: A standard corpus of files for testing purposes.

The set of 50 test files  derived from the logistic map are available for test purposes. Note that these are gziped - packed ASCII coded binary files comprising 10,000,000 bits.

The naming of the files is FILE# _  r-value _  T-entropy(Shannon normalised value of the entropy)

A further set of 100 files is provided here in gzipped and packed form:
The naming of these files is FILE# _  r-value _  T-entropy(Shannon normalised value of the entropy).pk.gz
 

If gunzip the files you need to further unpack the 8-bits per byte to get the individual bits of  the 10,000,000 bits strings.

Note that the information content of the packed string is very slightly different from the unpacked string .... something that needs to be taken into account if you are going to treat these as byte character files. The logistic map because of the period doubling ends up with cycles that may reduce the byte-character alphabet size to much less than the 256 possible characters that would ordinarily exist in a variety of strings.



Papers to read...

Relevant papers are available here:

"Compressor performance, absolutely!" Titchener 2002
  postscript, PDF
(This paper uses the above corpora to effectively analyse the absolute performance of  a number of popular compressors)

"Partition based entropies of  Deterministic and Stochastic maps"
    Ebeling, Steuer, Titchener, 2001  postscript, PDF

"Deterministic Chaos and Information Theory",
       Titchener  ebeling 2001 postscript, PDF

"A measure of information", Titchener 1999
  postscript, PDF

"Towards a calibrated Corpus for Compression testing"
    Titchener, Fenwick, Chen, 1998  postscript, PDF
 
 
 
 
 



Gallery of entropy pics.

The following images are of entropy surfaces computed for the logistic map using coarse graining in order to symbolically encode sampled time series, from which the T-entropy for the data is computed as a function of the bi-partition and control parameter, r. The graphcs is generated using a propriatory virtual 3D package written in C, and based on openGL calls.
 

View from above of the entropy surface computed for the logistic map.