User:Michellepoliseno

HW 5: 7.4.37 <= Let R be a commutative ring with 1 and the set of all non-units be M. We want to show that M is a Unique Maximal in R. Assume that M is not maximal. Then $$ \exists \ $$ I ideal in R such that M $$ \subset \ $$ I $$ \subset \ $$ R. And M $$ \ne \ $$ I $$ \ne \ $$ R. We know that 1 does not exist in M, because 1 is a unit. So if M is not equal to I, then $$ \exists \ $$ u $$ \in \ $$ I, where u is a unit. Then 1 $$ \in \ $$ I. But that implies I=R. But if I=R, then I is not a maximal idea. If I is not maximal, then that implies that M is maximal. Now assume that M is not unique. Then $$ \exists \ $$ M' such that M' is a maximal ideal in R. Then M' $$ \subset \ $$ R and M $$ \ne \ $$ R. So 1 $$ \notin \ $$ M'. Thus M' is the set of all non-units. That implies that M'=M. Therefore M is a unique maximal ideal in R.

Analysis:

3.20. Show that the sum and product of two simple functions are simple. Show that:

$$ \chi _{A\cap B} = \chi _A \cdot \chi _B $$

$$ \chi _{A\cup B} = \chi _A + \chi _B - \chi _A \cdot \chi _B $$

$$ \chi _{A^c} = 1 - \chi _A $$.

3.23. Prove Proposition 22 by establishing the following lemmas:

Proposition 22: Let $$ f\ $$ be a measurable function defined on an interval $$ [a,b]\ $$, and assume that $$ f\ $$ takes the values $$ \pm \infty $$ only on a set of measure zero. Then given $$ \varepsilon >0\ $$, we can find a step function $$ g\ $$ and a continuous function $$ h\ $$ such that $$ \left|{f-g}\right| < \varepsilon $$ and $$ \left|{f-h}\right| < \varepsilon $$ except on a set of measure less than $$ \varepsilon $$; i.e., $$ m \left\{{ x: \left|{f(x)-g(x)}\right| \ge \varepsilon }\right\} < \varepsilon $$ and $$ m \left\{{ x: \left|{f(x)-h(x)}\right| \ge \varepsilon }\right\} < \varepsilon $$. If in addition $$ m\le f\le M $$, then we may choose the functions $$ g\ $$ and $$ h\ $$ so that $$ m\le g\le M $$ and $$ m\le h\le M $$.

a.) Given a measurable function $$ f\ $$ on $$ [a,b]\ $$ that takes the values $$ \pm \infty $$ only on a set of measure zero, and given $$ \varepsilon > 0 $$, there is an $$ M\ $$ such that $$ \left|{f}\right| \le M $$ except on a set of measure less than $$ \frac{\varepsilon}{3} $$.

b.) Let $$ f\ $$ be a measurable function on $$ [a,b]\ $$. Given $$ \varepsilon > 0 $$ and $$ M\ $$, there is a simple function $$ \varphi $$ such that $$ \left|{f(x)-\varphi (x)}\right| < \varepsilon $$ except where $$ \left|{f(x)}\right| \ge M $$. If $$ m\le f\le M $$, then we may take $$ \varphi $$ so that $$ m\le \varphi \le M $$.

c.) Given a simple function $$ \varphi $$ on $$ [a,b]\ $$, there is a step function $$ g\ $$ on $$ [a,b]\ $$ such that $$ g(x) = \varphi (x) $$ except on a set of measure less than $$ \frac{\varepsilon}{3} $$. If $$ m\ge \varphi \ge M $$, then we can take $$ g\ $$ so that $$ m\ge g\ge M $$.

d.) Given a step function $$ g $$ on $$ [a,b] $$, there is a continuous function $$ h $$ such that $$ g(x) = h(x) $$ except on a set of measure less than $$ \frac{\varepsilon}{3} $$. If $$ m\ge g\ge M $$, then we may take $$ h $$ so that $$ m\ge h\ge M $$.

3.24. Let $$ f $$ be measurable and $$ B $$ a Borel set. Then $$ f^{-1} [B] $$ is a measurable set. (The class of sets for which $$ f^{-1} [E] $$ is measurable is a $$ \sigma $$-algebra.)

3.25. Show that if $$ f $$ is a measurable real-valued function and $$ g $$ a continuous function defined on $$ (-\infty, \infty )\ $$, then $$ g \circ f $$ is measurable.

$$ (g \circ f) ^{-1} \ $$ $$ \implies \ $$ $$ f^{-1} (g^{-1} (-\infty, \infty )) \ $$, we know that $$ (g^{-1} (-\infty, \infty )\ $$ is measurable since g is continuous on $$ (-\infty , \infty )\ $$ . Then by 3.24, we know that since f is measurable, $$ f^{-1}\ $$ is measurable. Thus $$ (g \circ f) ^{-1} \ $$ is measurable.

3.28. Let $$ f_1 $$ be the Cantor ternary function, and define $$ f $$ by $$ f(x) = f_1(x) + x $$.

a.) Show that $$ f $$ is a homomorphism of $$ [0,1] $$ onto $$ [0,2] $$.

b.) Show that $$ f $$ maps the Cantor set onto a set $$ F $$ of measure 1.

c.) Let $$ g = f^{-1} $$. Show that there is a measurable set $$ A $$ such that $$ g^{-1}[A] $$ is not measurable.

d.) Give an example of a continuous function $$ g $$ and a measurable function $$ h $$ such that $$ h \circ g $$ is not measurable. Compare with Problems 25 and 26.