International Journal of Theoretical and Applied Mathematics
Volume 2, Issue 2, December 2016, Pages: 28-34

Necessary and Sufficient Conditions of e-Separability

Maria A. Ivanchuk1, Igor V. Malyk2

1Department of Biological Physics and Medical Informatics, Bucovinian State Medical University, Chernivtsi, Ukraine

2Department of the System and Analysis and Insurance Financial Mathematics, Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine

Email address:

(M. A. Ivanchuk)
(I. V. Malyk)

To cite this article:

Maria A. Ivanchuk, Igor V. Malyk. Necessary and Sufficient Conditions of e-Separability.International Journal of Theoretical and Applied Mathematics Vol. 2, No. 2, 2016, pp. 28-34. doi: 10.11648/j.ijtam.20160202.11

Received: September 13, 2016; Accepted: October 28, 2016; Published: November 21, 2016


Abstract: In this paper we propose a new approach for solving the classification problem, which is based on the using e-nets theory. It is shown that for e-separating of two sets one can use their e-nets in the range space w.r.t. halfspaces, which considerably reduce the complexity of the separating algorithm for large sets’ sizes. The separation space which contains the possible values of e for e-nets of both sets is considered. The separation space is quasi-convex in general case. To check necessary and sufficient conditions of e-separability of two sets one can solve an optimisation problem, using the separation space as constraints. The lower bound of the separation space is convex for the exponential distribution and linear for the uniform distribution. So, we have convex and linear optimisation problems in these cases.

Keywords: e-nets, Separability, Quasiconvexity


1. Introduction

In 1987 D. Haussler and E. Welzl [1] introduced e-nets. Since that time e-nets are used in computational and combinatorics geometry.

Let  be a set (possibly infinite) and . The pair  is called a range space  with  its points and the elements of  its ranges. Typical examples of  range spaces in geometric context are set of halfspaces in , set of balls in , set of convex hulls in , axis parallel rectangles in the plane [2], set of d-dimensional simplexes in [3], triangles in the plane [4], set of a-fat wedges in the plain [5].

Numerous works study e-nets of one set [2-6]. B. Aronov et al. shown the existence of ε-nets of size  for planar point sets and axis-parallel rectangular ranges [2]. J. Kulkarni et al. get an e-net of size  for -fat wedges in  [5]. B .Gärtner proved the existing of e-nets of size , where  is Vapnik-Chervonenkis dimension [3]. Matousek J. et al. shown that disks and pseudo-disks in the plane as well as halfspaces in  allow e-nets of size only , which is best possible up to a multiplicative constant [6]. But, unfortunately, the analogous questions for higher-dimensional spaces remain open.

In this paper we will build e-nets of two sets for solving the classification problem.

Consider two sets  and  in Euclidian space . Consider the set  contains  points, set  contains  points. Let’s denote by    the convex hull of the set . Let .

Definition 1. Sets  and  are called e-separable if there exist subsets , , such that

(1)

and

(2)

The classification problem consists of finding the hyperplane , which separates  into two halfspaces  and , such that .

Definition 2. Hyperplane  is called e-separating for the sets and  if 

.

Consider an infinite range space , where  is the closed halfspaces in  bounded by hyperplanes. In [7] the following theorem was proved.

Theorem 1. A necessary and sufficient condition that two sets of points  and  are e-separable is there exist  and corresponding e-nets ,  in  such that

(3)

and

(4)

To find , which satisfy the inequality (3), let’s denote the separation space.

Definition 3. The set

(5)

is called the separation space for.

Let sets  and  are generated by the general populations  and  with distributions  and .

Definition 4. The set

(6)

is called the separation space for.

Consider the Euclidian space . In [8] the following lemma was proved.

Lemma 1. Let the inverse function  exist. Then the sets  and  are separated by the curve

(7)

Let’s consider the general case, when distribution functions don’t have the inverse functions in some points. We will use the generalized inverse [9].

Definition 5. For an increasing function   with  and , the generalized inverse

(8)

with the convention that . If  is a distribution function,  is also called the quantile function of .

In [8] the following lemma was proved.

Lemma 2. Sets  and  are separated by the line

(9)

For the line , which separates the sets   and  the following theorem was proved in [8].

Theorem 2. Let the following conditions

(1)  The sets  of size  are generated by the independent continuous random variables .

(2)  The sets  and  are separated by the line .

are satisfied. Then there exist the following equality

,

where

.

In this paper we will consider separation curve  for the sets ,  and it’s properties.

2. Results and Discussion

2.1. Quasiconvexity of the Separation Curve

Consider the function  where  is separation curve for the sets , . Let’s show that in the general case function  is quasiconvex [10].

Definition 6. Let  be the convex subset of . Function  is called quasiconvex, if for any  and  

.

Theorem 3. Function

is quasiconvex.

Proof. A continuous function  is quasiconvex if and only if at least one of the following conditions holds:  is nondecreasing;  is nonincreasing; there is a point  such that for ,  is nonincreasing, and for ,  is nondecreasing [10]. Let’s show that function  is nonincreasing.

(1)       Consider function . Function  is nondecreasing, so,  is also nondecreasing. Then  is nonincreasing. Since  is nondecreasing,  is nonincreasing.

(2)       Consider function . Function  is nondecreasing, so  is also nondecreasing. Then  is nonincreasing.

Since function  and  are nonincreasing,  is nonincreasing. Thus, Theorem 3 is proved.

In particular cases function  may be convex or linear. Consider corresponding examples.

2.2. Exponential Distribution

Let  two random variables which is exponentially distributed with parameters. Then according to Lemma 1 we have

.

Lemma 3. Let two random variables  exponentially distributed with parameters , then function , which separates sets  and  is convex.

Proof. Let’s consider the following two possible cases.

(1)   Let , then

We need to show that the function  is convex. Since

 as .

So, function  is convex as .

(2)   Let , then

Also, need to show that the function  is convex. Since

 as .

So, function  is convex as . Thus, Lemma 3 is proved.

Let  and . Function  is illustrated in the figure 1.

Fig. 1. Function  for exponential distribution.

2.3. Uniform Distribution

Let  two random variables which is  uniformly distributed with parameters  and . According to the  Lemma 1, the set  is lower bounded by the function

.

Lemma 4. Let two random variables  uniformly distributed with parameters  and , then function , which separates sets  and  is linear decreasing function.

Proof. For this reason consider the following two possible cases:

(1) Let for : , then

;

So,  is linear function. Since , function  is decreasing.

(2) Let for : , then

So,  is linear function. Since , function  is decreasing. Thus, Lemma 4 is proved.

Let  and . Function  is illustrated in the figure 2.

Fig. 2. Function  for uniform distribution.

2.4. Normal Distribution

Let  two random variables which is normally distributed with parameters  and . According to lemma 1 the set  is lower bounded by the function

,

where

.

Let  and . Function  is illustrated in the figure 3.

Fig.3. Function  for normal distribution.

2.5. Checking the Necessary and Sufficient Conditions for e-Separability

Obviously, for any  the equality (4) holds. So, to check the e-separability of the sets  and  it’s enough to check the existing such  that the inequality (3) holds. Consider the optimisation problem

(10)

(11)

Let  be the solution of the task (10), (11). If the equality (3) doesn’t hold for , it doesn’t hold for all , it means the sets  and  are not e-separable.  According to the Theorem 2, the condition (10) can be changed into 

(12)

According to the Theorem 3, in the general case, the  set  is quasiconvex. According to the Lemma 4, the set  is convex if the distribution is exponential. In this case the problem (10), (12) is the problem of convex programming. In case of uniform distribution, according to the Lemma 5, the task (10), (12) is the task of linear programming.

Let the point  is the solution of the optimisation problem (10), (12).

In the paper [8] the following theorem was proved.

Theorem 4. Let

(1)   have distribution , have distribution ,

(2)   be independent random variables,

(3)  Functions ,  have inverse and bounded densities  exist.

Then we obtain the weak convergence

,

where

, as ,

According to the theorem 4, the point  belongs to the neighbourhood

,

where ,  is probability, is t-distribution quantile for the corresponding probabilities in  (fig.4).

If in the boundary point  the condition (4) of the theorem 1 doesn’t hold, namely:

,

then the sets  and  are not e-separable.

Fig. 4. The solution of the optimisation problem.

Example 1. Consider two sets of points  and  generated by the uniformly distributed general populations  and . Let , , , , . Let’s verify the e-separability of the sets  and  as .

Build the set  (fig.5) and consider the task of linear programming

(13)

(14)

(15)

The solution of the linear programming problem (13)-(15) is the point  (fig.5). Let’s find the confidential interval for the solution of the problem

(16)

(17)

with probability .

According to the theorem 4

 as ,

so, the confidential interval for the problem (16), (17) solution degenerates to the point .

Fig.5. Lower bounds of the sets  and  and the linear programing problem’s solution the point

Let’s verify the condition (3) of the theorem 1

.

The condition (3) doesnt hold for , hence it doesn’t hold for all . So, the sets  and  are not e-separable as .

Example 2. Consider two sets of points  and  generated by the exponentialy distributed general populations  and . Let , , , . Let’s verify the e-separability of the sets  and  as .

Let’s build the set  (fig.6). Consider the problem of convex programming

(18)

(19)

(20)

The solution of the convex programming problem (18)-(20) is the point  (fig.6).

Fig.6. Lower bounds of the sets  and  and the convex programing problem’s solution the point

Let’s find the confidence interval for problem’s solution with probability  

(21)

(22)

According to theorem 4

 as ,

then

.

So, the confidence interval for the problem’s (18)-(20) solution is .

Let’s verify the condition (3) in the boundary point  

.

The condition (3) doesnt hold for , hence it doesn’t hold for all . So, the sets  and  are not e-separable as .

Example 3. Consider two sets of points  and  generated by the general populations . Let random variable  be uniformly distributed with the parameters ,  and random variable  be normally distributed with the parameters . Let , . Let’s verify the e-separability of the sets  and  as . Consider the problem of convex programming

(23)

(24)

(25)

The convex programming problems solution  is illustrated in the figure 7.

Fig. 7. Lower bounds of the sets  and  and the convex programing problem’s solution the point

Let’s find the confidence interval for problem’s solution with probability  

(26)

(27)

 as, hence the confidential interval degenerates to the point . Let’s verify the condition (3).

The condition holds. So, the sets  and  are e-separable as .

3. Conclusions

Two sets  and  are e-separable iff there exist their e-nets ,  in  such that conditions (3)-(4) hold. To verify the conditions (3)-(4) it is enough to solve the optimisation problem (10)-(11). If the solution of the problem (10)-(11) does not satisfy the condition (3), then the sets  and  are not e-separable. According to the theorem 2, one can use the theoretical set  as constraints for the optimisation problem (10)-(11). Three examples show verification of e-separability for uniform and exponential distributions.


References

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  2. Aronov B., Ezra E., Sharir M. "Small-size epsilon-nets for axis-parallel rectangles and boxes", Symposium on Theory of Computing, 2009, p.p. 639–648.
  3. Gärtner B., Hoffmann M. Computational Geometry, http://www.ti.inf.ethz.ch/ew/lehre/CG12/lecture/CG%20lecture%20notes.pdf.
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  6. Matousek J., Seidel R., Welzl E. "How to Net a Lot with Little:Smalle-Nets for Disks and Halfspaces"In Proc. sixth annual symposium on Computational geometry, 1990,  P.P. 16–22.
  7. Ivanchuk M.A., Malyk I.V. "Solving the Classification Problem Using ε-nets", Cybernetics and Systems Analysis, Vol.52, Iss.4 (2016), p.p. 613-622.
  8. M. A. Ivanchuk, I. V. Malyk "Using ε-Nets for Linear Separation of Two Sets in a Euclidean Space R d " , Cybernetics and Systems Analysis, Vol.51, Iss.6, 2015, P.P. 965-968. - DOI 10.1007/s10559-015-9789-7.
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