# 数学代写|离散数学作业代写discrete mathematics代考|MPCS50103

## 数学代写|离散数学作业代写discrete mathematics代考|Prime Number Theory

A positive integer $n>1$ is called prime if its only divisors are $n$ and 1 . A number that is not a prime is called composite.
Properties of Prime Numbers
(i) There are an infinite number of primes.
(ii) There is a prime number $p$ between $n$ and $n$ ! $+1$ such that $n0$, there exist $k$ consecutive composite integers).

Proof (i) Suppose there are a finite number of primes and they are listed as $p_1, p_2$, $p_3, \ldots, p_k$. Then consider the number $N$ obtained by multiplying all known primes and adding one. That is,
$$N=p_1 p_2 p_3 \ldots p_k+1$$

Clearly, $N$ is not divisible by any of $p_1, p_2, p_3, \ldots, p_k$ since they all leave a remainder of 1 . Therefore, $N$ is either a new prime or divisible by a prime $q$ (that is not in the list of $p_1, p_2, p_3, \ldots, p_k$ ).

This is a contradiction since this was the list of all the prime numbers, and so the assumption that there are a finite number of primes is false, and we deduce that there are an infinite number of primes.

Proof (ii) Consider the integer $N=n !+1$. If $N$ is prime then we take $p=N$. Otherwise, $\mathrm{N}$ is composite and has a prime factor $p$. We will show that $p>n$.

Suppose $p \leq n$ then $p \mid n !$ and since $p \mid N$ we have $p \mid n !+1$ and therefore $p \mid 1$, which is impossible. Therefore, $p>n$ and the result is proved.

Proof (iii) Let $p$ be the smallest prime divisor of $n$. Since $n$ is composite $n=u v$, and clearly $p \leq u$ and $p \leq v$. Then $p^2 \leq u v=n$ and so $p \leq \sqrt{n}$.

Proof (iv) Consider the $k$ consecutive integers $(k+1) !+2,(k+1) !+3, \ldots$, $(k+1) !+k,(k+1) !+k+1$. Then each of these is composite since $j \mid(k+1) !+j$ where $2 \leq j \leq k+1$.

## 数学代写|离散数学作业代写discrete mathematics代考|Algorithms

An algorithm is a well-defined procedure for solving a problem, and it consists of a sequence of steps that takes a set of values as input, and produces a set of values as output. It is an exact specification of how to solve the problem, and it explicitly defines the procedure so that a computer program may implement the algorithm. The origin of the word ‘algorithm’ is from the name of the 9th Persian mathematician, Muhammad al-Khwarizmi.

It is essential that the algorithm is correct and that it terminates in a reasonable time. This may require mathematical analysis of the algorithm to demonstrate its correctness and efficiency, and to show that termination is within an acceptable time frame. There may be several algorithms to solve a problem, and so the choice of the best algorithm (e.g. fastest/most efficient) needs to be considered. For example, there are several well-known sorting algorithms (e.g. merge sort and insertion sort), and the merge sort algorithm is more efficient $[\mathrm{\alpha}(n \lg n)]$ than the insertion sort algorithm $\left[0\left(n^2\right)\right]$.

An algorithm may be implemented by a computer program written in some programming language (e.g. C++ or Java). The speed of the program depends on the algorithm employed, the input value(s), how the algorithm has been implemented in the programming language, the compiler, the operating system and the computer hardware.

An algorithm may be described in natural language (care is needed to avoid ambiguity), but it is more common to use a more precise formalism for its description. These include pseudo code (an informal high-level language description); flowcharts; a programming language such as $\mathrm{C}$ or Java; or a formal specification language such as VDM or $\mathrm{Z}$. We shall mainly use a natural language and pseudo code to describe an algorithm. Among the early algorithms developed was an algorithm to determine the prime numbers up to a given number $n$, and Euclid’s algorithm for determining the greatest common divisor of two natural numbers. These are discussed below.

# 离散数学代写

## 数学代写|离散数学作业代写discrete mathematics代考|Prime Number Theory

(i) 有无限个质数。
(ii) 有一个㭌数 $p$ 之间 $n$ 和 $n !+1$ 这样 $n 0$ ， 存在 $k$ 连续复合整数）。

$$N=p_1 p_2 p_3 \ldots p_k+1$$

## 数学代写|离散数学作业代写discrete mathematics代考|Algorithms

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