Chapter 4: Problem 1
Show that \(t^{n}-1\) is divisible by \(t-1\).
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Chapter 4: Problem 1
Show that \(t^{n}-1\) is divisible by \(t-1\).
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L.ct \(F\) be a field. Show that the ideal \(\left(t_{1}, t_{2}\right)\) is not principal in the ring \(F\left[t_{1}, t_{2}\right]\). Similarly, show that \(\left(t_{1}, \ldots, t_{2}\right)\) is not principal in the ring \(F\left[t_{1+\cdots}, t_{n}\right]\)
Let \(f(X)=X^{3}+a_{1} X^{2}+a_{2} X+a_{3} .\) Show that the discriminant of \(f\) is $$ a_{1}^{2} a_{2}^{2}-4 a_{2}^{3}-4 a_{1}^{3} a_{3}-27 a_{3}^{2}+18 a_{1} a_{2} a_{3} . $$ [Reduce the question to the case of a polynomial \(Y^{3}+a Y+b\), and use the formula for this special case.]
Let \(R\) be a rational function over the field \(K\), and express \(R\) as a quotient of polynomials, \(R=g / f\). Define the derivative $$ R^{\prime}=\frac{f g^{\prime}-a f^{\prime}}{f^{2}} $$ where the prime means the formal derivative of polynomials as in the text. (a) Show that this derivative is independent of the expression of \(R\) as a quotient of polynomials, i.e. if \(R=g_{1} / f_{1}\) then $$ \frac{f g^{\prime}-a f^{\prime}}{f^{2}}=\frac{f_{1} g_{1}^{\prime}-\theta_{1} f_{1}}{f_{1}^{2}} $$ (b) Show that the derivative of rational functions satisfies the same rules as before, namely for rational functions \(R_{1}\) and \(R_{2}\) we have $$ \left(R_{1}+R_{2}\right)^{\prime}=R_{1}^{\prime}+R_{2}^{\prime} \quad \text { and } \quad\left(R_{1} R_{2}\right)^{\prime}=R_{1} R_{2}^{\prime}+R_{1}^{\prime} R_{2} $$ (c) Let \(x_{1} \ldots \ldots \alpha_{n}\) and \(a_{1} \ldots, a_{n}\) be elements of \(K\) such that $$ \frac{1}{\left(t-x_{1}\right) \cdots\left(t-x_{n}\right)}=\frac{a_{1}}{t-x_{1}}+\cdots+\frac{a_{n}}{t-x_{n}} $$ Let \(f(t)=\left(t-\alpha_{1}\right) \cdots\left(t-x_{n}\right)\) and assume that \(\alpha_{1}, \ldots, \alpha_{n}\) are distinct. Show that $$ a_{1}=\frac{1}{\left(x_{1}-\alpha_{2}\right) \cdots\left(\alpha_{1}-\alpha_{n}\right)}=\frac{1}{f^{\prime}\left(\alpha_{1}\right)} . $$
Let \(R=Z[t]\). Let \(p\) be a prime number. Show that \(t-p\) is a prime element in \(R\). Is \(t^{2}-p\) a prime element in \(R ?\) What about \(t^{3}-p ?\) What about \(t^{n}-p\) where \(n\) is a positive integer?
Let \(F\) be a field and let \(a: F[t] \rightarrow F[t]\) be an automorphism of the polynomial ring such that \(\sigma\) restricts to the identity on \(F\). Show that there exists elements \(a \in F, a \neq 0\), and \(b \in F\) such that \(\sigma t=a t+b\).
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