Sharif Digital Repository

Today there are many Internet sites which require only the entry by human users but unfortunately some computer softwares called bots are designed by some hackers to enter these sites and use their resources through false registration. As a result some systems named CAPTCHA have been introduced to tell apart human users and computer software. This paper introduces a new CAPTCHA method. In this method a simple mathematical problem is generated according to a predefined pattern but instead of some object's name, we put their images. Then the whole problem is saved and shown to the user inform of an image to be answered by him. But since answering this problem requires four abilities of... 

In pressurized water reactors, the fuel reloading problem has significant meaning in terms of both safety and economics. An optimal loading pattern is defined as a pattern in which the local power peaking factor (Pq) is lower than a predetermined value during one cycle and the effective multiplication factor (keff) is maximized to extract the maximum energy. This article presents a specialized genetic algorithm for loading pattern design. The tests on well-researched cases have shown that the genetic algorithm is capable of finding better loading patterns than solutions found by direct search methods. However, most of the previous researchers have considered simple fitness (cost) functions;... 

The endurance time (ET) method is a dynamic analysis in which structures are subjected to intensifying excitations, also known as ET excitation functions (ETEF). The ET method is a tool for structural response prediction. The main advantage of the ET method over conventional approaches is its much lower demand for computational efforts. The concept of acceleration spectra is used in generating existing ETEFs. It is expected that ETEF acceleration spectra increase consistently with time and remain proportional to a target spectrum. Nonlinear unconstrained optimization is commonly used to generate ETEFs. Generating new ETEFs is a complicated time-consuming mathematical problem. If the target... 

In this paper the problem of k out of n detection, between sequence of M random bits is addressed. The main application of the result is in a radar system, when we want to detect a target (with unknown time of arrival (TOA)), using binary integration. Another application is in ESM systems, when the system wants to detect the existence of a swept jammer or a gated noise jammer. But as any other mathematical problem, it may have some other applications. In this paper some simple equations for P fa and P d calculation of a detector which detects the existence of sequence of n ones between M random bits, is derived. These equations are then used to find the optimal detector structure in some... 

Heat exchanger network synthesis (HENS) is one of the most efficient process integration tools to save energy in chemical plants. In this work, a new optimization framework is proposed for the synthesis of HENS, based on a genetic algorithm (GA) coupled with a commercial process simulator through the ActiveX capability of the simulator. The use of GA provides a robust search in complex and nonconvex spaces of mathematical problems, while the use of a simulator facilitates the formulation of rigorous models for different alternatives. To include the most common heat exchanger structures in the model, a promising superstructure has been used. Allowing nonisothermal mixing of streams in the new...