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6 | 6 | \begin{enumerate} |
7 | 7 | \item{If $x,\,y\in X$ there is a unique $z\in X$ such that $x+z=y$.} |
8 | 8 | \item{ $0\cdot x=0=\alpha\cdot 0 \quad (\alpha\in\field, x\in X)$.} |
9 | | -\item{ $2A\subseteq A+A$.} |
| 9 | +\item{ $2A\subset A+A$.} |
10 | 10 | \item{ $A$ is convex if and only if $(s+t)A=sA+tA$ % |
11 | 11 | for all positive scalars $s$ and $t$.} |
12 | 12 | \item{ Every union (and intersection) of balanced sets is balanced.} |
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25 | 25 | \begin{enumerate} |
26 | 26 | %: (a) |
27 | 27 | \item Such property only depends on the group structure of $X$: Each $x$ in |
28 | | -$X$ has an opposite $\minus x$. Let $x'$ be any opposite of $x$, so that |
| 28 | +$X$ has an opposite $\minus x$. Let $x'$ be any opposite of $x$so that |
29 | 29 | ${x-x=0}=x+x'$. % |
30 | 30 | Thus, $\minus x +x -x = \minus x + x + x' $, % |
31 | 31 | which is equivalent to $\minus x = x'$. So is established the uniqueness of % |
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70 | 70 | \begin{align}\label{double lies in sum} |
71 | 71 | 2A = \{2x: x\in A \} |
72 | 72 | = \{x + x: x \in A \} |
73 | | - \subseteq \{ x + y : (x,\,y) \in A^2 \} |
| 73 | + \subset \{ x + y : (x,\,y) \in A^2 \} |
74 | 74 | = A+A |
75 | 75 | \end{align} |
76 | 76 | % |
|
79 | 79 | \item If $A$ is convex, then % |
80 | 80 | % |
81 | 81 | \begin{align} |
82 | | - A \subseteq \frac{s}{s+t} A + \frac{t}{s+t} A \subseteq A; |
| 82 | + A \subset \frac{s}{s+t} A + \frac{t}{s+t} A \subset A; |
83 | 83 | \end{align} |
84 | 84 | % |
85 | 85 | which is % |
|
104 | 104 | (or simply use the fact that $+$ is commutative!). |
105 | 105 | So ends the proof. % |
106 | 106 | %: (e) |
107 | | -\item Let $A$ range over $B$ a collection of balanced subsets, so that % |
| 107 | +\item Let $A$ range over $B$ a collection of balanced subsetsso that % |
108 | 108 | % |
109 | 109 | \begin{align} |
110 | | - \alpha \bigcap B \subseteq \alpha A \subseteq A \subseteq \bigcup B |
| 110 | + \alpha \bigcap B \subset \alpha A \subset A \subset \bigcup B |
111 | 111 | \end{align} |
112 | 112 | % |
113 | 113 | for all scalars $\alpha$ of magnitude $\leq 1$. % |
114 | | -The inclusion $\alpha \bigcap B \subseteq A$ establishes the first part. % |
| 114 | +The inclusion $\alpha \bigcap B \subset A$ establishes the first part. % |
115 | 115 | Now remark that % |
116 | 116 | % |
117 | 117 | \begin{align} |
118 | | - \alpha A \subseteq \bigcup {B} |
| 118 | + \alpha A \subset \bigcup {B} |
119 | 119 | \end{align} |
120 | 120 | % |
121 | 121 | implies % |
122 | 122 | % |
123 | 123 | \begin{align} |
124 | | - \alpha \bigcup {B} \subseteq \bigcup {B}; |
| 124 | + \alpha \bigcup {B} \subset \bigcup {B}; |
125 | 125 | \end{align} |
126 | 126 | which achieves the proof. % |
127 | 127 | % |
128 | 128 | %: (f) |
129 | | -\item Let $A$ range over $C$ a collection of convex subsets, so that % |
| 129 | +\item Let $A$ range over $C$ a collection of convex subsetsso that % |
130 | 130 | % |
131 | 131 | \begin{align} |
132 | | - (s+t) \bigcap C \subseteq s\bigcap C + t\bigcap C \subseteq sA + tA |
133 | | - \overset{(d)}{\subseteq} (s+t)A |
| 132 | + (s+t) \bigcap C \subset s\bigcap C + t\bigcap C \subset sA + tA |
| 133 | + \overset{(d)}{\subset} (s+t)A |
134 | 134 | \end{align} |
135 | 135 | % |
136 | 136 | for all positives scalars $\mathit{s}$, $\mathit{t}$. % |
|
159 | 159 | % |
160 | 160 | So, $x_1, x_2$ are now elements of the convex set $C_2$. % |
161 | 161 | Every convex combination of our $x_i$'s is then in % |
162 | | -$C_2 \subseteq \bigcup \Gamma$. Hence (g). % |
| 162 | +$C_2 \subset \bigcup \Gamma$. Hence (g). % |
163 | 163 | % |
164 | 164 | %: (h) |
165 | 165 | \item Simply remark that |
|
174 | 174 | \item Given any $\alpha$ from the closed unit disc, % |
175 | 175 | % |
176 | 176 | \begin{align} |
177 | | -\alpha(A+B)=\alpha A+ \alpha B \subseteq A+B. |
| 177 | +\alpha(A+B)=\alpha A+ \alpha B \subset A+B. |
178 | 178 | \end{align} |
179 | 179 | % |
180 | 180 | There is no more to prove: $A+B$ is balanced. % |
|
197 | 197 | To prove the lemma, let $\mathit{S}$ % |
198 | 198 | run through all nonempty subsets of $X$. % |
199 | 199 | First, assume that (i) holds: Clearly, every $S$ is convex balanced. % |
200 | | -Moreover, $S+S \subseteq S$. Conversely, $S = S + \{0\} \subseteq S + S $; % |
| 200 | +Moreover, $S+S \subset S$. Conversely, $S = S + \{0\} \subset S + S $; % |
201 | 201 | which establishes (ii). % |
202 | 202 | % |
203 | 203 | Next, assume (only) (ii): A proof by induction shows that % |
|
211 | 211 | Thus, % |
212 | 212 | % |
213 | 213 | \begin{align} |
214 | | - nS \overset{(\ref{induction nS})}{\subseteq} S |
215 | | - \subseteq n\,\lambda S |
216 | | - \subseteq n^2 S, |
| 214 | + nS \overset{(\ref{induction nS})}{\subset} S |
| 215 | + \subset n\,\lambda S |
| 216 | + \subset n^2 S, |
217 | 217 | \end{align} |
218 | 218 | % |
219 | 219 | since $S$ is balanced. % |
|
224 | 224 | Dividing the latter inclusions by $n$ shows that % |
225 | 225 | % |
226 | 226 | \begin{align} |
227 | | - S \subseteq \lambda S \subseteq nS \overset{(\ref{induction nS})}{\subseteq} S, |
| 227 | + S \subset \lambda S \subset nS \overset{(\ref{induction nS})}{\subset} S, |
228 | 228 | \end{align} |
229 | 229 | % |
230 | 230 | which is (iii). Finally, dropping (ii) in favor of (iii) leads to % |
|
245 | 245 | % |
246 | 246 | \begin{align} |
247 | 247 | \alpha S + \beta S = 0S + 0S\overset{(b)}{=} \{0\} |
248 | | - \overset{(b)}{=} 0S \subseteq S. |
| 248 | + \overset{(b)}{=} 0S \subset S. |
249 | 249 | \end{align} |
250 | 250 | % |
251 | 251 | Hence (i), which achieves the lemma's proof. % |
|
257 | 257 | First, remark that every member of $V$ is convex balanced: % |
258 | 258 | So is $I$ (combine (e) with (f)). % |
259 | 259 | % |
260 | | -Next, let $\mathit{Y}$ range over $V$, so that % |
| 260 | +Next, let $\mathit{Y}$ range over $V$so that % |
261 | 261 | % |
262 | 262 | \begin{align} |
263 | | - I + I \subseteq Y + Y \subseteq Y; |
| 263 | + I + I \subset Y + Y \subset Y; |
264 | 264 | \end{align} |
265 | 265 | % |
266 | 266 | which yields |
|
269 | 269 | I + I = I |
270 | 270 | \end{align} |
271 | 271 | % |
272 | | -(the fact that $I = I + \{0\} \subseteq I + I$ was tacitely used). % |
| 272 | +(the fact that $I = I + \{0\} \subset I + I$ was tacitely used). % |
273 | 273 | % |
274 | 274 | It now follows from the lemma's (ii) $\Rightarrow$ (i) that % |
275 | 275 | $I$ is a vector subspace of $X$. % |
|
279 | 279 | % |
280 | 280 | To show that $U$ is more specifically a vector subspace, % |
281 | 281 | we first remark that such total order implies that either % |
282 | | -$Z \subseteq Y$ or $Y \subseteq Z$, as $\mathit{Z}$ ranges over $V$. % |
| 282 | +$Z \subset Y$ or $Y \subset Z$, as $\mathit{Z}$ ranges over $V$. % |
283 | 283 | A straightforward consequence is that |
284 | 284 | % |
285 | 285 | \begin{align} |
286 | | - Y \subseteq Y + Z \subseteq Y\cup Z. |
| 286 | + Y \subset Y + Z \subset Y\cup Z. |
287 | 287 | \end{align} |
288 | 288 | % |
289 | 289 | Another one is that $Y \cup Z$ ranges over $V$ as well. % |
290 | 290 | Combined with the latter inclusions, this leads to % |
291 | 291 | % |
292 | 292 | \begin{align} |
293 | | - U \subseteq U + U \subseteq U. |
| 293 | + U \subset U + U \subset U. |
294 | 294 | \end{align} |
295 | 295 | % |
296 | 296 | It then follows from the lemma's (ii) $\Rightarrow$ (i) that % |
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