Counting Runs of Ones and Ones in Runs of Ones in
Binary Strings
Counting runs in binary strings
Frosso S. Makri, Zaharias M. Psillakis, Nikolaos Kollas
Departments of Mathematics and Physics
University of Patras
Patras, Greece;
Abstract—Consider a binary string (a symmetric Bernoulli sequence) of length . For a positive integer , we exactly enumerate,
in all possible binary strings of length , the number of all runs of 1s of length (equal, at least) and the number of 1s in all runs of 1s of
length at least . To solve these counting problems, we use probability theory and we obtain simple and easy to compute explicit formulae as
well as recursive schemes, for these potential useful in engineering numbers.
Keywords-runs; symmetric Bernoulli trials; probability theory; combinatorial problems
1. Introduction and Preliminaries
Nowadays, the increasing use of the computer science in
diverse applications including encoding, compression and
transmission of digital information calls for understanding the
distribution of runs of 1s or 0s. For instance, such knowledge
would help in analyzing, and comparing also, several
techniques used in communication networks (wired or
wireless). In such networks binary data, ranging from a few
bytes (e.g. e-mails) to many gigabytes of greedy multimedia
applications (e.g. video on demand), are highly processed. For
details, see [1-2] and the references therein.
Another area where the study of the distribution of runs of
1s and 0s has become increasingly useful is the field of
bioinformatics or computational biology. In particular,
molecular biologists examine tandem repeats among DNA
(Deoxyribonucleic acid) segments trying to specify how
probable are runs of matches, denoted as 1s, in adjacent
segments of a DNA sequence. See, e.g. [3-5].
In such applications, as the indicative ones mentioned
above, a key point is the understanding how 1s and 0s are
distributed and combined among the elements of a binary
sequence (finite or infinite, memoryless or not) and eventually
forming runs of 1s and 0s according to certain enumeration
rules (counting schemes). Each enumeration rule defines how
runs of same symbols (i.e. 1s or 0s) are formed and
consequently counted. A rule may depend on, among other
considerations, whether overlapping counting is allowed or not
as well as if the counting starts or not from scratch when a run
of a certain size has been so far enumerated. For extensive
reviews of the runs literature we refer to [6-8]. The topic is still
active and attractive too, because of the wide range of its
application in many areas of applied probability and
engineering including hypothesis testing, quality control,
system reliability and financial engineering. Some recent
contributions on the subject, among others, are the works of [9]
– [22].
Let be a sequence of binary (two-state) random
variables (RVs) taking on the values zero (0) or one (1) ordered
on a line. According to Mood’s [23] enumeration scheme a run
of 1s (1-run) is defined to be a sequence of consecutive 1s
preceded and succeeded by 0s or by nothing. The number of 1s
in a 1-run is referred to as its length (or size). For a positive
integer  let  denote the number of 1-runs of length
exactly in the first , binary trials. Following
Makri and Psillakis [19], we use the indicator functions
 ending at  
 with the convention . Consequently, the
statistic  can be expressed as 
 . The RV ,
which is a fundamental one in the run literature, besides its
independent merit, may be used for the representation of other
interesting statistics, too. Among them, the following two have
been frequently discussed in the literature and in particular in
financial engineering and bioinformatics [4, 17-18]. They refer
to 1-runs of length exceeding a positive integer , ,
in binary trials, and they are the number  of 1-runs of
length at least ; 
 , and the number  of 1s in
all 1-runs of length at least ;  
 An alternative
interpretation of  is that it denotes the sum of the lengths of
the 1-runs of length greater than or equal to  The statistics
 have been studied on binary sequences of
several internal structures by many researches who used
various methods. See, e.g. [1, 4, 9, 11-14, 17-19, 23-30].
In this brief note we show how someone can easily
enumerate explicitly the (total) number of occurrences of all 1-
runs associated with the first two mentioned statistics, as well
as, the (total) number of 1s according to the third one, in all
possible binary strings of length . Our approach is relied
on simple and efficient probabilistic arguments. It provides an
Open Journal of Applied Sciences
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alternative way to recapture explicit formulae for numbers
associated with 1-runs and it also establishes a new explicit
expression for the number of 1s in certain 1-runs. A unified
recursive scheme for these numbers is provided, too.
2. Main Results
Let 
stand for the RVs ,,  for  ,
respectively. The support (range set) of 
 
  
 . (1)
Next, we consider a sequence 
of length of
independent (i.e. derived by a memoryless source) and
identically distributed 0-1 RVs with a common probability of
1s  i.e.  , 
  Such a sequence, called a finite Bernoulli sequence,
is of particular importance in studies of applied probability
because of its simplicity, and also since it may be considered as
a special case of a sequence with dependent elements; e.g. a
Markovian or an exchangeable one.
For a Bernoulli sequence of length , let   
denote the probability mass function (PMF) of the RV 
; i.e.
 
 ,   
,  . (2)
Then, the expected value of 
  
   
 (3)
is given by (see Makri et al. [11])
 
   
  
 
 
   
In the sequel, we consider a symmetric () finite
Bernoulli sequence (i.e. a finite binary string) of length for
which we obtain our main results. Since the cardinality of a
proper sample space is (i.e. there are binary strings that
are equally likely to occur) the classical definition of
probability implies that
  
 
. (5)
The numbers 
 ,  
, for  admit the
following interpretation: (i) 
 ,   is the number of
all binary strings of length with exactly  
 , 1-runs of length [exactly (),
at least ()] , among all the possible binary strings of
length , . (ii) 
 is the number of all binary
strings of length with exactly  n, 1s
contained in all 1-runs of length at least , among all the
possible binary strings of length , .
Simple explicit expressions of 
 (not repeated here) are
given in Makri and Psillakis [19]. The authors provided an
explicit formula of 
 , for  in terms of binomial
coefficients. Their method is based on the solution of a
combinatorial problem; specifically, the allocation of balls into
cells under certain constrains (see Lemma 2.2 of [11]). An
explicit expression of 
 , in terms of binomial coefficients
too, is given by Sinha and Sinha [1] who used a generating
function approach. The latter expression contains an additional
sum; therefore it may be evaluated slower computationally than
that provided in [19].
The numbers 
 allow us to establish (we do not actually
need their specific expressions according to the new proposed
approach) respective numbers referring to all possible
binary strings of length . They are defined as
, (6)
i.e. 
 is the total number of occurrences of all 1-runs of
length [exactly (), at least ()] , and the total
number of 1s in all 1-runs of length at least [], in all
possible binary strings of length , .
Since 
  
, hence
by (5) and (6), 
 
. Therefore (4) implies
   
  
  
 
  
Readily, by symmetry, 
 provides the respective
numbers associated with 0-runs and 0s in 0-runs.
By (7) it is noted that for a fixed , 
,  
decreases exponentially as increases, and for a fixed 
, as , it holds
 
 
 
 
 
Furthermore, 
 
 , since   ,
Table I presents the three numbers 
 in binary
strings of length  bits,  , and for . The
entries of the table confirm the previously noted behavior of
the depicted numbers.
Sinha [31] was the first who addressed the usefulness of the
number 
 and also provided its formula. Then, Sinha and
Sinha [1] obtained an explicit expression of 
 whereas
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Makri and Psillakis [19] derived the same formula for it and
they also established an explicit expression of 
. In both
papers ([1] and [19]) the approach was relied on the definition
of 
 via 
 , . Recently, Sinha and Sinha [2]
reestablished 
 solving explicitly a recursive generation
scheme for it.
The proposed, in the present note, approach is a new one
and treats under the same frame all the numbers 
 in a
simple, unified and systematic way. Accordingly, by (7) we
effortless get a recursive scheme for 
,  . It gives a
way to generate 
 from 
 and it offers further insight in
understanding the interdependencies among the studied
numbers. Specifically, it holds
    
 
 
 
  
with 
   . We note that
for the particular case  we capture by the relevant entries
of (9) and (7), Theorems 2 and 3 of [2], respectively.
 AND
3. Conlusions
In this note we stated three run statistics which are
important in many areas of applied probability. We defined
them on a binary (0-1) sequence, and we then provided
explicitly their mean values for a Bernoulli sequence. After that,
we considered binary strings (symmetric Bernoulli sequences)
and we showed how the analytic expressions of the means of
these RVs provide eventually the respective explicit
expressions of three numbers studied recently by different
methods. Finally, as a byproduct of our approach, we proposed
a unified recursive scheme which clarifies further the
interdependencies among these numbers. The examined
numbers are potential useful in many engineering applications
like the ones mentioned briefly in the Introduction. Early
results are encouraging in this direction.
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