USE OF COMPUTER EXPERIMENTS TO STUDY THE QUALITATIVE BEHAVIOR OF SOLUTIONS OF SECOND ORDER NEUTRAL DIFFERENTIAL EQUATIONS

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1 USE OF COMPUTER EXPERIMENTS TO STUDY THE QUALITATIVE BEHAVIOR OF SOLUTIONS OF SECOND ORDER NEUTRAL DIFFERENTIAL EQUATIONS Seshadev Padhi, Manish Trivedi and Soubhik Chakraborty* Department of Applied Mathematics Birla Institute of Technology, Mesra, Ranchi , India ABSTRACT Use of computer experiments have become very popular in the recent past with VLSI design, combustion, experimental studies on algorithmic complexity to name only a few of the vast number of its applications. Since the root of a differential equation is strictly determinable for a given set of parameters, it makes it plain that if we run the code, which solves the characteristic equation, for the same set of parameters invariably we shall get identical results. In the present paper, we are motivated to realize the deterministic response of a neutral differential equation as the outcome of a stochastic process and conduct factorial experiments to obtain much valuable information on the behavior of the solution with emphasis on both the main treatments (parameters) and their interactions AMS Subject classification: 34 K 15, 62 K 15 Key words: neutral differential equation; factorial experiments Acknowledgement: This work is supported by Birla Institute of Technology, Mesra, Ranchi, India. s: ses_2312@yahoo.co.in (S. Padhi) manish_trivedi1976@yahoo.co.in (M. Trivedi), soubhikc@yahoo.co.in (S.Chakraborty): *corresponding author 1

2 1 INTRODUCTION In this paper, an attempt has been made to study the qualitative behavior of solution of the second order neutral differential equations of the form y // (t) = p 1 y / (t) + p 2 y / (t-τ) + q 1 y(t) + q 2 y(t-τ) + r 1 y // (t-τ), t 0 (1) with the initial condition y(t)=φ(t), -τ t 0 where p 1, p 2, q 1, q 2, and r 1 and τ are real constants and τ > 0 using factorial experiments. Equation of the form (1) has been studied theoretically by many authors on the oscillation and non-oscillation, stability and asymptotic stability of the solution of the equation. For instance, see [1, 4] and the references cited therein. Setting y(t) = e λt, we obtain the following characteristic equation of (1) Set F(λ) = -λ 2 λ 2 = p 1 λ + p 2 λe -λτ + q 1 + q 2 e -λτ + r 1 λ 2 e -λτ (2) + p 1 λ + p 2 λe -λτ + q 1 + q 2 e -λτ + r 1 λ 2 e -λτ We have made a computational study of the characteristic equation (2) to analyze the behavior of solution of (1) and have studied the same for simultaneous variation of the associated parameters in (2). In a recent work [2], factorial experiments have been successfully used in the context of computer experiments. The motivation for the present work on computer experiments has come from the work of Sacks et. al. [3]. 2. EXPERIMENTAL FINDINGS BY PROGRAMMING NEWTON- RAPHSON METHOD TO SOLVE (2) Our purpose is to study the behavior of the solution of the neutral differential equation for simultaneous variation of the associated parameters p 1, p 2, q 1, q 2, τ and r 1 each within certain limit. Using the concept of factorial experiments as used in statistics we shall, taking the parameters as treatments, study which main effects and interaction effects are significant in a comparative sense. For this purpose we shall conduct computer experiments. A computer experiment is a series of runs of a code for various inputs. A deterministic computer experiment is one in which re-running the code with identical inputs leads to identical responses. Most computer experiments are deterministic and this is true irrespective of whether the response is the output or a complexity (such as time in [2]). However by realizing the response as the outcome of a stochastic process it is possible to derive some potential benefits such as reducing the cost of prediction (this means predicting the response for untried inputs for which it may be computationally cumbersome to run the code). This was in fact the motivation of Sacks and others in ref. [3]. Here also the situation is deterministic as the root is strictly determinable for a given set of parameters and fixed τ so that if we run the code for the same set of parameters invariably we shall get identical results. Following the principle advocated by Sacks[3] 2

3 we are motivated to realize the deterministic response of our neutral differential equation as the outcome of a stochastic process and conduct factorial experiments to obtain much valuable information on the behavior of the solution with special emphasis on both the main treatments (parameters) and their interaction effects. However four different values of tau are used to generate some noise. This helps us in conducting the factorial experiment with four blocks (replicates). The three degrees of freedom arising from the four blocks are added to the error degrees of freedom so that treatments can be compared with greater precision. This is precisely what is done in the MINITAB statistical package where we have analyzed the results of the computer experiments. Experimental study ( 2 5 Factorial experiment) We shall conduct a 2 5 factorial experiment (five factors each at two levels).using standard notation, in table 1.1 to 1.4 the parameters p 1, p 2, q 1, q 2 and r 1 (which we shall denote as p1, p2, q1, q2, r1) correspond to a, b, c, d, e respectively. Levels are 1 and 10 for each. Presence of the alphabet (a-e) indicates the corresponding parameter (treatment) is at the second level while absence indicates it is at first level. Table 2 gives MINITAB Results for 2 5 factorial experiment. Table 1.1 Block 1 Observations of root for τ =1 Treatment combination observation Treatment combination observation a b ab c ac bc abc d ad bd abd cd acd bcd abcd e ae be abe ce ace bce abce de ade bde abde cde acde bcde abcde

4 Table 1.2 Block 2, Observations of root for τ =1.5; Treatment combination observation Treatment combination observation a b ab c ac bc abc d ad bd abd cd acd bcd abcd e ae be abe ce ace bce abce de ade bde abde cde acde bcde abcde Table 1.3 Block 3 Observations of root for τ =2; Treatment combination observation Treatment combination observation a b ab c ac bc abc d ad bd abd cd acd bcd abcd e ae be abe ce ace bce abce de ade bde abde cde acde bcde abcde

5 Table 1.4 Block 4 Observations of root for τ =2.5; Treatment combination observation Treatment combination observation a b ab c ac bc abc d ad bd abd cd acd bcd abcd e ae be abe ce ace bce abce de ade bde abde cde acde bcde abcde Table 2 MINITAB Results for 2 5 factorial experiment General Linear Model: observation versus level a, level b,... Factor Type Levels Values level a fixed level b fixed level c fixed level d fixed level e fixed Analysis of Variance for observant, using Adjusted SS for Tests Source DF Seq SS Adj SS Adj MS F P level a level b level c level d level e level a*level b level a*level c level a*level d

6 level a*level e level b*level c level b*level d level b*level e level c*level d level c*level e level d*level e level a*level b*level c level a*level b*level d level a*level b*level e level a*level c*level d level a*level c*level e level a*level d*level e level b*level c*level d level b*level c*level e level b*level d*level e level c*level d*level e level a*level b*level c* level d level a*level b*level c* level e level a*level b*level d* level e level a*level c*level d* level e level b*level c*level d* level e level a*level b*level c* level d*level e Error Total REMARK In addition to the p-values in table 2, we also have the analysis for 5% level of significance:- F(1, 80) at 5% level=3.96 while F(1, 100) at 5% level=3.94. Hence F(1, 95) at 5% level is some value lying in the interval [3.94, 3.96]. We did not feel it necessary to interpolate or regress to estimate this F value since none of the calculated values of F falls in this interval. All calculated F values are either below 3.94 or above 3.96 and hence clearly either insignificant or significant at 5% level of significance respectively. 6

7 The general conclusions are as under:- Single Factor effects: All are highly significant Two factor effects: ac, ce are insignificant. bd, be, cd are significant but not highly. All others are highly significant. Three factor effects: acd is insignificant. abd, ace, cde are significant but not highly. All others are highly significant. Four factor interactions: acde is significant but not highly. Others are highly significant. Five Factor interaction: The sole five factor interaction abcde is significant certainly though not highly. The QBASIC program, which generated the data given in table 2, is as follows:- REM root of a neutral differential equation by Newton Raphson method CLS INPUT "enter accuracy"; e INPUT "enter p1,p2,q1,q2,r1,tau in that order"; p1, p2, q1, q2, r1, tau INPUT "enter initial guess"; l 10 fl = (-1) * l * l + p1 * l + (r1 * l * l + p2 * l + q2) * EXP((-1) * l * tau) + q1 fld = (-2) * l + p1 + (2 * r1 * l + p2) * EXP((-1) * l * tau) - tau * (r1 * l * l + p2 * l + q2) * EXP((-1) * l * tau) lnew = l - fl / fld IF ABS(l - lnew) >= e THEN l = lnew: GOTO 10 PRINT "root="; lnew END REMARK This program was run in our system for four different values of τ corresponding to the four blocks and fixing values of the remaining parameters, for each τ value, according to the treatment combination. Thus we fixed tau = 1 for each observation in block 1 (table 1.1). Now, the observation for treatment combination ab for example was obtained by running the code for p1 = 10, p2 = 10, q1 = 1, q2 = 1, r1 = CONCLUDING REMARKS We conclude that it is quite possible to create an interface between researches in statistical approaches to computer experiments with that in differential equations to get much valuable information on the behavior of solution of a differential equation. 7

8 References [1] A.F. Yenicerioglu, The behaviour of solutions of second order delay differential Equations, Jour. of Math. Analysis and Appl. (2006), in press, doi: /j.jmaa [2] A. Kumari and S. Chakraborty, Software Complexity: A Statistical Case Study Through Insertion Sort, Applied Math. and Compu., vol. 190(1), p [3] J. Sacks, W. Welch, T. Mitchell, H. Wynn, Design and Analysis of Computer Experiments, Statistical Science, 4(4), 1989 [4] I. Gyori and G. Ladas, Oscillation Theory of Delay Differential Equation, Clarendon Press, Oxford,