Vol. 7 Issue 1

M.A. El-Affendi
Consistent measurements have shown that the service and interarrival time distributions for most computer resources are not exponential. The immediate consequence of this finding is that the popular Markovian models such as the M/M/I, M/M/C etc. do not accurately represent the underlying computer resources and therefore may not be totally reliable in the performance analysis of computer systems. Computer resources may be more appropriately represented by nonexponential models where both the service and interarrival times may be of a general type. Although this seems to be a natural resort, in practice analysis refrain from the use of non-exponential models because they are hard to solve and may not lead to useful solutions. This paper is an attempt to remove some of the difficulties associated with the analysis of non-exponential computer models. It is shown that the spectral solution of Lindley's integral equation with the help of Rouche's theorem may easily be used to obtain exact solutions for the non-exponential models of computer performance analysis. Several examples are used to illustrate the method. These include the E2/H2/I2, the H2/H2/I and E2/E2/I models for which exact performance measures are given for the first time, to the best of our knowledge.

Dieter Jungnickel
Let A be an affine plane of even order which admits a representation by an affine difference set D in an abelian group G (relative to N), say G = H ⊕ N. We discuss various hyperovals of A related to this representation: (-D + y) ⋃ (∞) is an oval with nucleus y (assuming w.l.o.g. D= 2D), and the sets H + n (n ⋲ N) (which form a partition of A\ (∞)) are ovals with common nucleus ∞. In case A=AG (2, 2°), all these ovals are in fact conics (for which we give explicit equations). In particular, the points of AG (2, 2°)\{(0, 0)} can be partitioned into 2°-1 conics with common nucleus (0, 0); moreover, there are commuting cyclic groups H and N such that H acts regularly on each of these conics whereas N acts regularly on the set of all these conics.

A. Majid Hamid, C.M. Ashraf and Attaur-Rehman
Hexane extract of the air-dried whole plant (Fagonia cretica Linn) afforded a crystalline compound, m.p. 75- 76 °C, which has been identified to be docosyl docosanoate. The molecular formula of this ester has been determined mass spectrometrically and by combustion analysis, while the structure has been assigned on the basis of I.R. and NMR spectroscopic techniques. The compound on alkaline hydrolysis yielded docosanoic (behenic) acid and 1-docosanol, which confirmed the structure of this ester

E.M. Abdelrahman, A.I. Bayoumi, and H.M. El-Araby
The frequency responses of the all three-dimensional second derivative coefficient sets derived by Rao et al. (1970), along with that of their simple formula and the theoretical response of the second derivative operation, using an infinite number of points average approach, are presented. The frequency responses of the coefficient sets reveal: (a) the superiority of the coefficient sets derived following Peters' approach over those sets derived following Elkins' approach, (b) the superiority of the coefficient sets derived with a weightage of l/r^4 to all circles over those derived with a weightage of l/r^2 or without any weightage to all circles, (c) that many coefficient sets derived following Peters' approach give more accurate results than the simple formula, and also (d) the coefficient sets derived with preference to central point give better results than those derived with no preference to the central point. However, for the calculation of the second derivative, two new weight coefficient sets which use the least possible number of circles for obtaining average values and at the same time yield good results are developed by making use of Richardson's improvement formula of the derivative. We also present a comparative picture of the frequency responses of the derived sets, along with that of the best set of derived Rao et al.

El-Sayed A.A. Youssef and Mohamed I.H. El Anbaawy
The oncolytic and oolitic limestones within the Jurassic Amran Group of Yemen Arab Republic (Y.A.R) display characteristics depositional and diagenetic structures. The oncoids have four types of laminations; (1) micritic, (2) grumose, (3) organodetrital-bearing and (4) clastic bearing laminations. Most of these laminae are concentric and continuous suggesting continued movements or agitated conditions during deposition. Some laminae are discontinuous and show local growth reflecting short periods of mild agitation or quiet conditions. The porosity of the studied facies is the result of early and late diagenetic history. It includes primary pore spaces represented by fenestral vugs and secondary pore spaces resulting from the dissolution of carbonate grains as well as cement. Cementation and compaction are the main factors reducing the porosity of these sediments. Compaction due to burial is represented by concavo-convex and sutured contacts and microstyloites.

Saad Khorfan
Hydrodesulfurization of gas-oil is a very important process in Syrian refining industry. This product contains high percentage of sulfur. It is used wildly for heating and transport. The presence of sulfur causes serious pollution and corrosion problems. This study investigates the influence of various variables on the hydrodesulfurization of heavy Syrian gas oil over Cobalt Molybdenum catalyst. The results obtained showed an increase in conversion with the increase in temperature in the range 360 - 420°C. The reaction was found to follow a second order kinetics. Pressure increases the conversion up to a certain level and then the influence is less marked.

E.M. Hassan and H.M. Hassan
Both the residence time of water in the Arabian Gulf defined as the length of time required for the volume of water entering the Gulf through the Strait of Hormuz to completely equal the volume of water in the Gulf and the renewal period defined as the length of time required for the volume of new water from the Gulf of Oman to equal the volume of water in the Gulf are the shortest for any body of water of similar dimensions. Important among the factors contributing to these periods are the tidal exchange and the excess evaporation. Tidal exchange was modelled by using the four components M2, S2, K1 and O1. The excess evaporation was estimated using meteorological data and river discharge. It is found that tides are potentially able to account for the shortness of the periods. The residence time was found to be less than one year, while the renewal time was found to be between 1.5 and 4.5 years with a mean of about three years

Mozahim I. Azawi
Experimental results on transient phenomena in semiconductor lasers are reported. A travelling-wave rate equation model for the investigation of the non-linear dynamics and chaos in semiconductor lasers is presented. The model includes the effects of defects which act as optically saturable absorbing centres, external cavity optical feedback, and the coefficient of spontaneous emission coupled into the lasing modes. Numerical results and analytical solutions confirm that the increase of density of saturable absorbers decreases the onset frequency of pulsations. Furthermore, these results provide strong evidence of chaotic dynamics in semiconductor lasers corporated with external cavity.

H.A. Al-Kahtani
The locally grown varieties of peanut, corn, and sesame seeds were evaluated for physical measurements and proximate composition. The extracted oils were analyzed for physico-chemical characteristics and fatty acid composition. Soundness and shell proportions of all peanuts met grade U.S. No. 1. The highest oil and protein contents were found in Baladi (El-Gasim and TAI-NAN-4 peanuts, respectively. Corn kernels significantly differed not only in length, weight and volume but also in oil, protein and ash contents. Dark sesame seeds were heavier and had significantly higher oiland lower protein contents than light-colored seeds. Generally, the physico-chemical characteristics of crude peanut, corn and sesame oils were within the range of the AOAC recommended standards. GLC showed number of fatty acids present in crude oils. The percentage of each fall in the range adopted by FAO/WHO committee on fats and oils. Oleic and linoleic were the principle unsaturated fatty acids while palmitic was the main saturated fatty acid.

Ghandorah, M.O.
Drought tolerance of twenty two durum wheat varieties were determined under field condition at the Agricultural Experiment Station of King Saud University, near Riyadh, Saudi Arabia. The varieties were grown under three water regimes. These regimes were achieved by irrigating the soil at 80, 60 and 40% of the available moisture. Increasing the available moisture percent increased grain yield, days to heading and the grain filling rate. However, the grain filling period was not affected. The interactions between water regimes and varieties were significant for grain yield and grain filling rate in the first season only. Significant differences were detected among varieties for all the studied characters. Relative drought susceptibility coefficient was calculated with varieties No. 20 and 22 being identified as having tolerance and higher grain yield.