Chapter 14: Problem 18
For \(p\) a prime determine all elements \(a \in \mathbf{Z}_{p}\) where \(a^{2}=a\).
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Chapter 14: Problem 18
For \(p\) a prime determine all elements \(a \in \mathbf{Z}_{p}\) where \(a^{2}=a\).
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Let \(R=M_{2}(\mathbf{Z})\) and let \(S\) be the subset of \(R\) where $$ S=\left\\{\left[\begin{array}{cc} x & x-y \\ x-y & y \end{array}\right] \mid x, y \in \mathbf{Z}\right\\} . $$ Prove that \(S\) is a subring of \(R\).
Give an example of a ring with eight elements. How about one with 16 elements? Generalize.
a) In how many ways can one select two positive integers \(m, n\), not necessarily distinct, so that \(1 \leq m, n \leq 100\) and the last digit of \(7^{m}+3^{n}\) is \(8 ?\) b) Answer part (a) for the case where \(1 \leq m, n \leq 125\). c) If one randomly selects \(m, n\) [as in part (a)], what is the probability that 2 is now the last digit of \(7^{m}+3^{n}\) ?
Find the multiplicative inverse of each element in \(\mathbf{Z}_{11}, \mathbf{Z}_{13}\), and \(\mathbf{Z}_{17}\).
Let \(R\) be a ring with ideals \(A\) and \(B\). Define \(A+B=\\{a+b \mid a \in A, b \in B\\}\). Prove that \(A+B\) is an ideal of \(R\). (For any ring \(R\), the ideals of \(R\) form a poset under set inclusion. If \(A\) and \(B\) are ideals of \(R\), with glb \(\\{A, B\\}=A \cap B\) and lub \(\\{A, B\\}=A+B\), the poset is a lattice.)
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