Grey Accumulation Generation Relational Analysis Model for Nonequidistance Unequal-Length Sequences and Its Application

As research is required on nonequidistance unequal-length sequences, so grey accumulation generation relational analysis model based on grey exponential law (AGRA) for nonequidistance unequal-length (NDUL) sequences is put forward in this paper. The original data is accumulated generation firstly and the generation sequences are simulated. Then the generation rate is established as the ratio of the tangent slope and the mean of the simulation function. Furthermore, the dynamic similarity of change trend of the original time sequences is characterized by the proximity of generation rate sequences. Meanwhile, properties of AGRA model for nonequidistance unequal-length sequences are discussed. The new relational analysis model is available for equal interval sequences, nonequidistance sequences, sequenceswhich have relationship before transformation and sequenceswhich have relationship after accumulation; therefore, theAGRAmodel has expanded the scope of application of grey relational analysis. Lastly, factors which affect the amount of passenger cars in China are sorted using AGRAmodel for NDUL sequences. This application is presented to illustrate the effectiveness and practicality of the proposed model.


Introduction
Grey relational analysis (GRA) has emerged as an effective model to measure the degree of correlation between sequences by calculating their similarity or proximity.Some research on GRA and its application has been done, such as the improvement and development of the existing GRA [1][2][3][4][5], extension of objects of studying [6], parallelism and consistency [7], consistency for affine transformation [8], and similarity and proximity of GRA [9].
GRA has been employed in various fields where they have produced promising results due to not limited data distribution and quantity.On the theoretical side, GRA plays its advantage during combination of clustering [10], decision making [11][12][13][14], and evaluating [15].There are also a range of practical or industrial applications employing GRA, for instance, in the period of Chinese regional economic discrepancy [16], affecting factors of service brand [17], company financial performance [18], software effort estimation, energy consumption [19], Chinese economic development and energy consumption [20], residential energy consumption and renewable energy utilization [21], sustainability indicator of renewable energy system [22], maximum peak discharge at upper stream [23], and track association algorithm for distributed multitarget system [24].As a summary, Liu et al. review the research approaches of GRA modeling systematically [25].Yin conducts a bibliometric study on publication and citation patterns of GRA published from 1996 to 2010 through a systemic search using the ISI webbased databases; results demonstrate that there has been a substantial increase in the number of peer-reviewed papers on GRA indexed by the ISI Web of Knowledge [26].
From the view of grey systems, although the objective system looks complex, it always has the overall function and certain inherent law.There is a method called grey generation which can explore the real law of the original data.Accumulation generation is an effective kind of grey generation, which whitens grey process and inherent development tendency of accumulation process.Original data may have no regular raw seemingly but emerge integral characteristics by accumulation generation [27].Because the economic system, ecological system, and agricultural system could be treated as 2 Mathematical Problems in Engineering generalized energy system, and the accumulation of energy follows the exponential rule, so accumulation generation has extensive adaptability and has been applied into grey forecasting models.Grey forecasting models based on grey exponential law are employed in a range of applications [28][29][30].
The existing GRA model examines geometric similarity or proximity of data directly or after dimensionless without considering other kinds of law, and research on GRA for nonequidistance unequal-length (NDUL) sequences is required, so AGRA for NDUL sequences based on grey exponential law will be put forward in this paper.Generation rate for NDUL sequences will be established.Then the dynamic change trend similarity of the original time series will be characterized by the proximity of generation rate sequence.Properties of AGRA for NDUL sequences will also be studied.Lastly, an application will be researched using the model proposed in this paper.
The paper is structured as follows.In the next section, AGRA for NDUL sequences is provided.Properties of AGRA for NDUL sequences are researched in Section 3.An application using the model is furnished in Section 4. In Section 5, appropriate conclusions are drawn.

Establishment of Grey Generation Rate
Relational Analysis Model for Nonequidistance Unequal-Length Sequences

Generation of Equidistance Equal-Length Sequences from
Nonequidistance Unequal-Length Sequences.Suppose that  0 (0)  (0) .The time intervals of new sequences are one and lengths are .
| 0 () −   ()| reflects the proximity of change trend at time  between  0 (0) and   (0) , so the closer development tendency at time  of  0 (0) and   (0) is, the smaller | 0 () −   ()| is, meanwhile, the greater the relational coefficient of  0 (0) and   (0) is, and vice versa.The constriction of function for the relational coefficient is consistent with the meaning of grey relational analysis.Definition 7. Grey relational degree of AGRA between   and   is constructed as The grey relational coefficient of AGRA between sequences reflects the degree of consensus of sequences with change trend at one moment, and the grey relational degree of AGRA between sequences is the mean of the grey relational coefficient, so the grey relational degree of AGRA between sequences is measurement of consensus of sequences with development tendency for the whole time.

The
Step of AGRA Model for NDUL Sequences.The steps to obtain the order of sequences by AGRA model for NDUL sequences are concluded as follows.
(1) Make qualitative analysis on system characteristics sequence  0 to confirm system behavioral sequences   .
(2) Get the accumulation generation sequences of  0 and   by (1).

Properties of AGRA Model for NDUL Sequences
Theorem 8. AGRA model for NDUL sequences has the following properties.
(2) Uniqueness and Independence.  is unique for fixed two sequences and is not influenced by other sequences.
(3) Proximity.The closer the development tendency of sequences is, the greater   is.
There is no sense to research correlation between these sequences.So   ̸ = 0.
(2) According to the definition of the grey relational coefficient and grey relational degree of AGRA,   is unique for fixed  0 (0) and   (0) and is not influenced by other sequences.
(4) Transitivity can be known by Properties 2 and 3.
So,   0 () =  0 (),   Therefore, the grey relational coefficients of generation rate between  0 (0) and   (0) will remain the same after  0 (0) and   (0) are divided by the initial value or the mean value, or multiplied; in other words, the grey relational coefficients of AGRA will not be affected by dimensionless so do the grey relational degree of AGRA.Another conclusion can be got; the AGRA model for NDUL sequences is suitable for both positive sequences and negative sequences.
Proof.The generation rates of  0 (1) and   (1) at time  are As As  can be ignored when compared with   and  0 (1) , and the derivative of  is infinitesimal, so Therefore, the AGRA model for NDUL sequences is stable for noise.

Case Study
Traffic jam is becoming one of the most difficult problems in which cities are facing as the urban space is expanding and the population is accumulating.Traffic accidents, air pollution, noise pollution, resource shortages, and other issues which are brought by traffic jam are hindering the global economy and even threatening human survival; that is why traffic jam is one of the most focused on problems for academic organizations and governments.It is significant to sort factors which affect the amount of commercial passenger cars for traffic management and congestion controlling.The road mileage, turnover passengers on highway, and the number of civil aircrafts are selected as affecting factors according to the existing literatures and expert advices.The original data of affecting factors are in Table 1.
Due to the limitation of data source, the numbers of commercial passenger cars and civil aircrafts in 2006 and 2007 are lacking.Common GRA models fail to handle this situation.In order to make full use of existing data, the AGRA model for NDUL sequences is required to rank factors which affect the number of commercial passenger cars.The parameters during the mathematical calculation are provided in Table 2.The grey generation rate degrees of incidence and the ranking list are in Table 3.
As shown in Table 3, the number of civil aircrafts is more affected by requirement of society than social capacity.The driving action of requirement of society is greater than the restriction of social capacity in front of the increasing of cars.

Conclutions and Future Studies
AGRA model for NDUL sequences is established in this paper.The original data is accumulated firstly and the curve is fitted by optimized GM (1,1) model for nonequidistance sequences; then the fitting equation is obtained.Generation rate is established as the ratio of the tangent slope and the mean of the fitting equation.The similarity of dynamic change trend of the original time series is characterized by the proximity of generation sequences.The AGRA model for NDUL sequences is available for both equal interval sequences and nonequidistance sequences.Meanwhile, the new model can handle sequences which have relationship before transformation and sequences which have relationship after accumulation.Accumulation generation reflects development tendency of data and expands the scope of application of grey relational analysis.Meanwhile, properties of AGRA model for NDUL sequences are researched.It is shown that AGRA model for NDUL sequences is not limited to positive sequences.In the last section, factors which affect the number of passenger cars in China are ordered using the model proposed in this paper.It is illustrated that the proposed model is effective and practical.
Because the economic system, ecological system, and agricultural system could be treated as generalized energy system, the accumulation of energy follows the exponential rule which is used to establish AGRA model.Meanwhile, data missing is common in practical problem so that NDUL sequences are general.In conclusion, AGRA model for NDUL sequences can be applied but not limited to economic system, ecological system, and agricultural system problems with NDUL sequences.
There are some potential studies that can be done in the future according to the ideas presented in this paper, including conducting a new AGRA model based on other kinds of GRA models; in this way, more applications of other kinds of AGRA models can be determined.The AGRA model for NDUL sequences can also be combined with clustering methods and decision making methods in the presence of NDUL sequences.

Table 1 :
The original data of factors which affect the number of commercial passenger cars.
As initial sequences in AGRA model for NDUL sequences are accumulation generated before taken into the model, and accumulation generation has noise reduction effect, so the model reduces the impact of noise.Stability and validity for noise interference in this model will be demonstrated as follows.Let  =   +  be the sequence which has noise,   the sequence without noise, and  the noise; then

Table 2 :
The parameters during the mathematical calculation.

Table 3 :
The grey relational degree of AGRA and the ranking of factors which affect the number of commercial passenger cars.