Problem 3 - Abstract Algebra: Theory and Applications

Question

Find all of the abelian groups of order 720 up to isomorphism.

Step-by-Step Solution

Answer

Question: List all abelian groups of order 720 up to isomorphism. Answer: The 10 abelian groups of order 720 up to isomorphism are: 1. \(Z_{2^4} \times Z_{3^2} \times Z_5\) 2. \(Z_{2^3} \times Z_2 \times Z_{3^2} \times Z_5\) 3. \(Z_{2^2} \times Z_{2^2} \times Z_{3^2} \times Z_5\) 4. \(Z_{2^2} \times Z_2 \times Z_2 \times Z_{3^2} \times Z_5\) 5. \(Z_2 \times Z_2 \times Z_2 \times Z_2 \times Z_{3^2} \times Z_5\) 6. \(Z_{2^4} \times Z_{3} \times Z_3 \times Z_5\) 7. \(Z_{2^3} \times Z_2 \times Z_3 \times Z_3 \times Z_5\) 8. \(Z_{2^2} \times Z_{2^2} \times Z_3 \times Z_3 \times Z_5\) 9. \(Z_{2^2} \times Z_2 \times Z_2 \times Z_3 \times Z_3 \times Z_5\) 10. \(Z_2 \times Z_2 \times Z_2 \times Z_2 \times Z_3 \times Z_3 \times Z_5\)

Step 1: Prime Factorization of 720

Find the prime factorization of 720. We get \(720 = 2^4 \cdot 3^2 \cdot 5^1\).

Step 2: Use the Fundamental Theorem of Finite Abelian Groups

According to the Fundamental Theorem of Finite Abelian Groups, every finite abelian group is isomorphic to a direct product of cyclic groups of prime-power order. Therefore, we can break down the group into products whose orders multiply to 720. For example, a group of order \(720=2^4 \cdot 3^2 \cdot 5^1\) can also be seen as a product of cyclic groups, such as \(Z_2 \times Z_2 \times Z_2 \times Z_2 \times Z_3 \times Z_3 \times Z_5\).

Step 3: Find All Possible Combinations of Cyclic Groups

We will now find all possible combinations of cyclic groups with order 720. For each prime factor, we can combine their powers in any partition. The partitions of the powers are as follows: \(2^4\): \((4)\), \((3,1)\), \((2,2)\), \((2,1,1)\), \((1,1,1,1)\). \(3^2\): \((2)\), \((1,1)\). \(5^1\): \((1)\). Now, we can create all possible combinations of the partitions of the powers: 1. \((4) \times (2) \times (1)\) 2. \((3,1) \times (2) \times (1)\) 3. \((2,2) \times (2) \times (1)\) 4. \((2,1,1) \times (2) \times (1)\) 5. \((1,1,1,1) \times (2) \times (1)\) 6. \((4) \times (1,1) \times (1)\) 7. \((3,1) \times (1,1) \times (1)\) 8. \((2,2) \times (1,1) \times (1)\) 9. \((2,1,1) \times (1,1) \times (1)\) 10. \((1,1,1,1) \times (1,1) \times (1)\)

Step 4: Write Down Every Abelian Group up to Isomorphism

Now, we write down every abelian group corresponding to the combinations found in step 3: 1. \(Z_{2^4} \times Z_{3^2} \times Z_5\) 2. \(Z_{2^3} \times Z_2 \times Z_{3^2} \times Z_5\) 3. \(Z_{2^2} \times Z_{2^2} \times Z_{3^2} \times Z_5\) 4. \(Z_{2^2} \times Z_2 \times Z_2 \times Z_{3^2} \times Z_5\) 5. \(Z_2 \times Z_2 \times Z_2 \times Z_2 \times Z_{3^2} \times Z_5\) 6. \(Z_{2^4} \times Z_{3} \times Z_3 \times Z_5\) 7. \(Z_{2^3} \times Z_2 \times Z_3 \times Z_3 \times Z_5\) 8. \(Z_{2^2} \times Z_{2^2} \times Z_3 \times Z_3 \times Z_5\) 9. \(Z_{2^2} \times Z_2 \times Z_2 \times Z_3 \times Z_3 \times Z_5\) 10. \(Z_2 \times Z_2 \times Z_2 \times Z_2 \times Z_3 \times Z_3 \times Z_5\) So, these are the 10 abelian groups of order 720 up to isomorphism. Note that each of these groups are indeed abelian and have an order of 720, as required.

Key Concepts

Prime Factorization

Prime factorization is the process of breaking down a number into its prime components. For the number 720, this means finding the smallest prime numbers that can be multiplied together to equal 720. This factorization is crucial as it lays the groundwork for further analysis of mathematical structures, such as abelian groups in our example.

When we factorize 720, we see it can be expressed as the product of powers of primes:

• 720 = 2^4 \( \cdot \) 3^2 \( \cdot \) 5^1

Understanding the prime decomposition of a number allows us to explore how many distinct structures, like groups, can be formed. This decomposition also enables us to apply several important theorems in group theory, for instance, how groups can be broken into simpler components.

Fundamental Theorem of Finite Abelian Groups

The Fundamental Theorem of Finite Abelian Groups is a cornerstone of group theory. It states that every finite abelian group is isomorphic to a direct product of cyclic groups of prime-power order. That simply means any finite abelian group can be represented by combining simpler structures, like Lego blocks, where each block is a cyclic group of a size that is a power of a prime number.

For the abelian groups of order 720, we apply this theorem by considering the prime factorization of 720, namely 2^4, 3^2, and 5^1. Each of these can be associated with cyclic groups, such as \( Z_2 \), \( Z_3 \), and \( Z_5 \). Conceptually, it allows us to have more manageable parts that come together to form the complex whole.

Through this theorem, we identify the number of possible group structures by looking at all the combinations of these cyclic groups of orders arising from our prime factorization, leading to various isomorphism classes.

Isomorphism

In mathematics, an isomorphism is a critical concept that highlights when two structures can be viewed as essentially the same, despite perhaps appearing different at first glance. With groups, if two groups are isomorphic, they have the same structure, meaning you can map one to the other perfectly.

For the abelian groups of order 720, describing groups "up to isomorphism" means we list representatives of each group type or structure rather than every name or form of the group. It simplifies classification as we seek structures that behave similarly.

• This approach allows us to not repeat groups that are essentially the same, despite potential differences in notation or appearance.

By looking at groups up to isomorphism, students and mathematicians can focus on understanding the fundamental similarities between groups, aiding more in-depth exploration and application of group theory principles.

Cyclic Groups

Cyclic groups are the basic building blocks discussed in the Fundamental Theorem and are groups generated by a single element. Essentially, they are groups where every element can be expressed as some power of a particular ("generating") element.

For abelian groups, cyclic groups are especially useful as they can represent more complex groups in simpler terms, making these groups easier to study and understand. For example, a group like \( Z_8 \) represents a cyclic group of order 8, meaning it wraps around itself every 8 elements.

In the context of our exercise, different combinations of cyclic groups of orders given by prime powers form the abelian groups. Cyclic groups of smaller prime power orders combine to emulate, or even construct, larger abelian groups of order 720 in different distinct forms, making them incredibly versatile and important in mathematical theory and practice.