_{1}

^{*}

For characterization of negative exponential distribution one needs any arbitrary non-constant function only in place of approaches such as identical distributions, absolute continuity, constant regression of order statistics, continuity and linear regression of order statistics, non-degeneracy etc. available in the literature. Path breaking different approach for characterization of negative exponential distribution through expectation of non-constant function of random variable is obtained. An example is given for illustrative purpose.

Knowing characterizing property may provide unexpectedly accurate information about distributions and one can recognize a class of distributions before any statistical inference is made. This feature of characterization of probability distributions is peculiar to characterizing property and attracted attention of both theoretician and applied workers but there is no general theory of it.

Various approaches were used for characterization of negative exponential distribution. Among many other people, Fisz [

In this research note section 2 is devoted for characterization based on identity of distribution and equality of expectation function randomly variable for a negative exponential distribution with probability density function (pdf).

where are known as constants, is positive absolute continuous function and is everywhere differentiable function. Since derivative of is positive, the range is truncated by from left.

Theorem 2.1 Let be a random variable with distribution function. Assume thatis continuous on the interval, where. Let and be two distinct differentiable and intregrable functions of X on the interval where and moreover be non constant. Then

is the necessary and sufficient condition for pdf of to be defined in (1.1).

Proof Given defined in (1.1), for necessity of (2.1) if is such that where is differentiable function then

Differentiating with respect to on both sides of (2.2), replacing for and simplifying one gets

which establishes necessity of (2.1). Conversely given (2.1), let be such that

which can be rewritten as

which reduces to

Hence

Since is increasing integrable and differentiable function on the interval with the following identity holds

Differentiating with respect toand simplifying (2.8) after taking as one factor, (2.8) reduces to

where is a function of only derived in (2.3) and is a function of and only derived in (2.7).

Since be increasing integrable and differentiable function on the interval where and since is positive intregrable function on the interval where with and integrating (2.7) over the interval on both sides, one gets (2.7) as

and

.

Substituting in derived in (2.10), reduces to defined in (1.1) which establishes sufficiency of (2.1).

Remark 2.1 Using derived in (2.3), the given in (1.1) can be determined by

and pdf is given by

where is decreasing function for with such that it satisfies

Illustrative Example: Using method described in the remark characterization of negative exponential distribution through survival function is illustrated.

To characterize pdf defined in (1.1) one needs any arbitrary non-constant function of which should only be differentiable and integrable.