代数和数论 Topics in Algebra and Number Theory|MATH 8845Boston College Assignment

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Algebra is a branch of mathematics that deals with the manipulation and properties of abstract objects, such as numbers, symbols, and equations. It includes the study of algebraic structures, such as groups, rings, and fields, and their properties and applications.

Number theory is a branch of mathematics that deals with the properties of numbers, particularly integers. It includes the study of prime numbers, divisibility, Diophantine equations, and modular arithmetic, among other topics.

Algebra and number theory are closely related, as algebraic structures such as groups, rings, and fields are often used in the study of number theory. For example, the study of prime numbers can be approached using algebraic techniques such as group theory, and the study of modular arithmetic relies heavily on properties of rings and fields. Many problems in number theory also have algebraic formulations and solutions.

代数和数论 Topics in Algebra and Number Theory|MATH 8845Boston College Assignment

问题 1.

Let $m=3^n p$ for some integer $n>0$ and prime $p$. Suppose $m$ is perfect.
(a) Prove that if $p \neq 2$ then $3^{n+1}-1=2 p$ and $p+1=2 \cdot 3^n$.

证明 .

Since $m$ is a perfect number, it must be of the form $m=2^{k-1}(2^k-1)$ for some positive integer $k$. Therefore, we have

$$m = 2^{k-1}(2^k-1) = 3^n p.$$

Since $p$ is prime and $p\neq 2$, we have two cases:

Case 1: $p\equiv 1\pmod 3$

In this case, we have $3^n \mid 2^{k-1}$, which implies $k-1\geq n$. Furthermore, since $p\equiv 1\pmod 3$, we have $p+1\equiv 2\pmod 3$, which implies $3\mid p+1$. Thus, we have

$$m = 2^{k-1}(2^k-1) \geq 2^{n}(2^{n+1}-1) > 2^n(2\cdot 3^n -1) = 2p,$$

which contradicts the fact that $m$ is perfect. Therefore, this case cannot occur.

Case 2: $p\equiv 2\pmod 3$

In this case, we have $3^n \mid 2^k-1$, which implies $k\geq n+1$. Furthermore, since $p\equiv 2\pmod 3$, we have $p+1\equiv 0\pmod 3$, which implies $3^{n+1}\mid p+1$. Thus, we have

$$m = 2^{k-1}(2^k-1) \geq 2^{n}(2^{n+1}-1) = 3^n(3^{n+1}-2) > 3^n(3^n\cdot 2 – 2) = 2\cdot 3^{2n} > 2p,$$

which again contradicts the fact that $m$ is perfect. Therefore, this case cannot occur either.

Therefore, there is no prime $p\neq 2$ that can make $m$ a perfect number.

Note: If $p=2$, then $m=2^{2n+1}-2^n$, and it is known that this is a perfect number if and only if $2^n$ is an odd prime (known as Mersenne prime).

问题 2. Let $m=3^n p$ for some integer $n>0$ and prime $p$. Suppose $m$ is perfect. Prove that $p=2$.

证明 .

Since $m$ is perfect, we know that the sum of its divisors (excluding itself) is equal to $m$. That is, if $d_1,d_2,\ldots,d_k$ are the positive divisors of $m$ (excluding $m$ itself), then we have $$d_1+d_2+\cdots+d_k=m.$$ Now let $d$ be any positive divisor of $m$. Then $d$ can be written as $3^ap^b$ for some nonnegative integers $a$ and $b$ with $0\leq b\leq 1$. (Note that $b=0$ is possible because $m$ need not be odd.) Since $d$ divides $m$, we have $3^ap^b\mid 3^n p$, which implies $3^a\mid 3^n$. Therefore, $a\leq n$. It follows that $d\leq 3^n p$.

Now consider the sum of the divisors of $m$ that are divisible by $3$. These are precisely the divisors of the form $3^ap$ with $1\leq a\leq n$. There are $n$ such divisors, each of which is at most $3^n p$, so the sum of these divisors is at most $n\cdot 3^n p$. Therefore, the sum of the divisors of $m$ that are not divisible by $3$ is at least $$m-(n\cdot 3^n p)=(3^n- n\cdot 3^n)p.$$ Since $m$ is perfect, this sum must be equal to $m$. That is, $$(3^n-n\cdot 3^n)p=3^n p+2^k,$$ where $k$ is a nonnegative integer and $2^k$ is the sum of the divisors of $m$ that are equal to $2^j$ for some $j\geq 1$. Since $p$ is prime, it follows that $p$ divides $2^k$. However, $p$ is odd because $p\neq 2$, so $p$ and $2$ are relatively prime. Therefore, we must have $p=2$.

问题 3.

Let $m=3^n p$ for some integer $n>0$ and prime $p$. Suppose $m$ is perfect. Prove that $m=6$.

证明 .

Let $d(m)$ denote the sum of the divisors of $m$. If $m$ is perfect, then $d(m)=2m$.

Since $m=3^n p$, the sum of the divisors of $m$ is given by $(1+3+\cdots+3^n)(1+p)$, which is equal to $\frac{3^{n+1}-1}{2}(p+1)$.

Thus, we have the equation $\frac{3^{n+1}-1}{2}(p+1)=2(3^n p)$.

Rearranging terms, we get $3^{n+1}p-3^n-1=0$.

Now, we observe that if $n\geq 2$, then $3^{n+1}p-3^n-1 \equiv 2 \pmod{3}$, which means that $3^n p$ cannot be perfect. Therefore, we must have $n=1$.

Plugging in $n=1$, we get $3p-2=0$, which implies $p=2/3$, a contradiction since $p$ is prime.

Therefore, there is no such $m$ such that $m=3^n p$ and $m$ is perfect.

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