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\centerline{\bf HW \#6}
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\centerline {\bf Math 506A}
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\noindent 1. (20 points) DF, Exercise 10, p. 811.
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\noindent 2. DF, Exercise 11, p. 811.  
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\noindent 3.  DF, Exercise 19, p. 812.
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\noindent 4. Let $A$ be the algebra over $\Bbb Q$ generated by two elements $x,y$ subject to the relations $x^2=-3,y^2=-7,xy=-yx$.  Show that $A$ is a division ring, by imitating the proof that the ring $H$ of quaternions is a division ring.
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