Chapter 6: Problem 19
Let \(R\) be a ring with at least two elements such that for every nonzero element \(a \in R\) there exists a unique element \(b \in R\) with \(a b a=a\). Show that (a) \(R\) has no zero divisors. (b) \(b a b=b\) (c) \(R\) has unity.
Short Answer
Expert verified
(a) Ring \(R\) has no zero divisors. (b) Element \(b\) satisfies \(bab = b\). (c) Ring \(R\) has a unity.
Step by step solution
01
Understanding Zero Divisors
Recall that a zero divisor in a ring is a nonzero element \(a\) for which there exists a nonzero element \(c\) such that \(ac = 0\). We will show that \(R\) has no zero divisors by assuming otherwise and reaching a contradiction.
02
Prove Part (a) - No Zero Divisors
Suppose for a contradiction that \(a\) and \(c\) are nonzero elements in \(R\) with \(ac = 0\). By hypothesis, there exists a unique \(b\) such that \(aba = a\). Consequently, \(ab = 1\) because if \(ab\) were zero, then \(a(ab)a = 0a = 0\), contradicting \(a(ab)a = a\). Hence, \(ab eq 0\). Thus, \(ac = 0\) implies that there are no nonzero divisors, proving no zero divisors exist.
03
Prove Part (b) - Verify the Property
Given that \(aba = a\), we also have \(bab\) to find. Multiply the equation \(aba = a\) on both sides by \(b\) to get \(abab = ab\). But since \(ab = 1\), \(ab = aba\), and thus \(babab = baba = b\). It follows that \(bab = b\).
04
Prove Part (c) - Existence of Unity
To show that \(R\) has a unity element, consider any nonzero element \(a\). Let \(b\) satisfy \(aba = a\). We want to show there exists an element \(e\) in \(R\) such that \(ae = ea = a\) for all \(a \in R\). Use \(a(ab) = a\) to deduce that \(ab = e\) serves as the identity for any arbitrary element shown by \(e \, e' = a, \, e'e = a\) for \(e,\, e' \), and \(e = e'\) proving that every element has an associate. All elements have a distinguished domain, thus \( R \) has a unity element.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Zero Divisors
In ring theory, zero divisors are elements that can multiply with a non-zero element to yield zero. Let's explore this concept in a friendly manner. Say you have a non-zero element \( a \); if there exists another non-zero element \( c \) such that \( ac = 0 \), then \( a \) and \( c \) are zero divisors. However, in the ring \( R \) given in our exercise, for every non-zero element \( a \), there exists a unique \( b \) such that \( aba = a \). This condition implies \( ab eq 0 \) because if \( ab = 0 \), then \( a(ba)a = 0 \), contradicting \( aba = a \). Therefore, \( R \) cannot have any zero divisors, as that would contradict the unique element property of the ring.
Unity Element
A unity element, also known as a multiplicative identity, is a special element in a ring that leaves other elements unchanged when multiplied. In simpler terms, this is like multiplying by 1 in a set of integers, where you have \( 1 \times n = n \times 1 = n \). In our ring \( R \), for each element \( a \), there exists a unique \( b \) such that \( aba = a \). By utilizing this relationship, we show that there is an element \( e \) such that for every element \( a \), the equation \( ae = ea = a \) holds.
- Start by considering \( ab \) which acts akin to an identity since \( a(ab) = a \).
- Because this holds for any non-zero \( a \), \( ab \) effectively becomes a unity for that element.
Unique Element Property
This fascinating aspect of a ring refers to each non-zero element having a singular associate, \( b \), that fulfills the equation \( aba = a \). The idea here is beautifully tied to uniqueness; no different \( b \) could satisfy \( aba = a \) for the same \( a \), due to the uniqueness condition. This property means that for each \( a \), the operation "sandwiching" with \( b \) returns \( a \) back to its original form, almost like a perfect reflection. It forms a foundational aspect of proving our points about zero divisors and the unity element. It adds a layer of structure to the ring by ensuring predictable behavior of elements within the ring and eliminating contradictions that may otherwise arise in systems without such a property.
Algebraic Proofs
Algebraic proofs are the structured, logical steps taken to establish truths within mathematical structures. Here, each proof demonstrates a specific property of the ring \( R \) under examination:
- Zero Divisors: By contradiction, you show no zero divisors can exist if for every element \( a \) in ring, unique \( b \) satisfies \( aba = a \).
- Unity Element: By examining the relationship \( aba = a \) and deducing the existence of \( ab \) acting as an identity, the proofs reinforce the existence of a unity element.
- Uniqueness: The unique association of each element to \( b \) forms the bedrock for many consecutively stacked deductions.