# Information about Functional Analysis - Part 19

• Title: Hölder’s Inequality

• Series: Functional Analysis

• YouTube-Title: Functional Analysis 19 | Hölder’s Inequality

• Bright video: https://youtu.be/yIXahhfRbTc

• Dark video: https://youtu.be/pjKPOxmGSUo

• Timestamps 00:00 Introduction 01:42 Hölder’s Inequality 02:19 Young’s Inequality 05:11 Proof
• Subtitle in English

1 00:00:00,550 –> 00:00:02,289 Hello and welcome back to

2 00:00:02,299 –> 00:00:03,589 functional analysis.

3 00:00:03,710 –> 00:00:04,929 And as always, I want to

4 00:00:04,940 –> 00:00:06,320 thank all the nice people

5 00:00:06,329 –> 00:00:07,590 that support this channel

6 00:00:07,599 –> 00:00:09,039 on study or paypal.

7 00:00:09,729 –> 00:00:11,689 In today’s part 19, we

8 00:00:11,699 –> 00:00:13,420 will go back and prove Hölder’s

9 00:00:13,430 –> 00:00:14,369 inequality.

10 00:00:15,250 –> 00:00:16,530 There are a lot of different

11 00:00:16,540 –> 00:00:17,930 inequalities named after

12 00:00:17,940 –> 00:00:19,350 Hölder and indeed, they are

13 00:00:19,360 –> 00:00:20,309 all related.

14 00:00:20,319 –> 00:00:21,850 But here we look at the simplest

15 00:00:21,860 –> 00:00:23,639 one, it’s the one that

16 00:00:23,649 –> 00:00:24,979 holds in FN.

17 00:00:24,989 –> 00:00:26,739 And for P greater than one

18 00:00:27,450 –> 00:00:29,190 for each such number P, there

19 00:00:29,200 –> 00:00:30,870 is a Hölder conjugate which

20 00:00:30,879 –> 00:00:32,119 we call P prime.

21 00:00:32,909 –> 00:00:34,080 The definition is what you

22 00:00:34,090 –> 00:00:35,479 should really memorize.

23 00:00:35,490 –> 00:00:37,020 The reciprocals should add

24 00:00:37,029 –> 00:00:37,610 to one.

25 00:00:38,180 –> 00:00:39,729 This is an important relation

26 00:00:39,740 –> 00:00:41,130 because every time we use

27 00:00:41,139 –> 00:00:42,750 P prime, this is what we

28 00:00:42,759 –> 00:00:44,529 mean instead of P

29 00:00:44,540 –> 00:00:46,099 prime, a lot of people just

30 00:00:46,110 –> 00:00:46,830 use Q.

31 00:00:47,630 –> 00:00:48,979 However, here I want to use

32 00:00:48,990 –> 00:00:50,790 Q as another variable to

33 00:00:50,799 –> 00:00:52,590 tell you again what the P

34 00:00:52,599 –> 00:00:53,380 norm was.

35 00:00:54,020 –> 00:00:55,240 Or today, I would better

36 00:00:55,250 –> 00:00:57,049 say the Q norm of a vector

37 00:00:57,060 –> 00:00:57,450 X.

38 00:00:58,250 –> 00:01:00,080 It’s defined by the sum over

39 00:01:00,090 –> 00:01:01,569 the entries in the absolute

40 00:01:01,580 –> 00:01:03,049 value to the power q.

41 00:01:03,709 –> 00:01:05,370 And then we take the q-th root

42 00:01:05,379 –> 00:01:06,290 of the whole thing.

43 00:01:07,069 –> 00:01:08,470 And this is what we do for

44 00:01:08,480 –> 00:01:09,980 all q between one and

45 00:01:09,989 –> 00:01:11,769 infinity including one.

46 00:01:12,470 –> 00:01:14,269 Now Hölder inequality will

47 00:01:14,279 –> 00:01:15,849 connect the P norm the P

48 00:01:15,860 –> 00:01:17,550 prime norm and the one

49 00:01:17,559 –> 00:01:19,489 norm indeed, it’s one

50 00:01:19,500 –> 00:01:20,839 nice short formula.

51 00:01:20,849 –> 00:01:22,160 You always should remember.

52 00:01:22,989 –> 00:01:24,510 I will now use a possibly

53 00:01:24,519 –> 00:01:26,110 strange notation to help

54 00:01:26,120 –> 00:01:27,699 you to remember the important

55 00:01:27,709 –> 00:01:29,610 inequality putting two

56 00:01:29,620 –> 00:01:31,250 vectors together just

57 00:01:31,260 –> 00:01:33,110 denotes a new vector which

58 00:01:33,120 –> 00:01:34,949 has the product in the components.

59 00:01:35,239 –> 00:01:36,930 So X to Y two in the

60 00:01:36,940 –> 00:01:38,559 second component and

61 00:01:38,569 –> 00:01:39,849 XN times Y.

62 00:01:39,860 –> 00:01:41,580 And in the last component,

63 00:01:41,940 –> 00:01:43,760 then for all vectors X and

64 00:01:43,769 –> 00:01:45,650 Y we have that the one norm

65 00:01:45,660 –> 00:01:47,440 of the vector XY

66 00:01:47,940 –> 00:01:49,430 is less or equal than the

67 00:01:49,440 –> 00:01:51,309 P norm of X times the P

68 00:01:51,319 –> 00:01:52,720 prime norm of Y.

69 00:01:53,379 –> 00:01:54,830 And that’s what we call a

70 00:01:54,839 –> 00:01:56,029 less inequality.

71 00:01:56,580 –> 00:01:57,720 Often you see it written

72 00:01:57,730 –> 00:01:59,199 with the sums, but I think

73 00:01:59,209 –> 00:02:00,910 it’s better to write it immediately

74 00:02:00,919 –> 00:02:02,599 in this way, on the one

75 00:02:02,610 –> 00:02:04,269 hand, it’s easier to remember.

76 00:02:04,360 –> 00:02:05,680 And on the other hand, we

77 00:02:05,690 –> 00:02:06,919 will generalize the whole

78 00:02:06,930 –> 00:02:07,860 thing later on.

79 00:02:08,470 –> 00:02:09,899 Indeed, there we will have

80 00:02:09,910 –> 00:02:11,199 the whole inequality for

81 00:02:11,210 –> 00:02:13,160 functions defined on an abstract

82 00:02:13,169 –> 00:02:13,929 measure space.

83 00:02:14,429 –> 00:02:15,479 However, that’s not what

84 00:02:15,490 –> 00:02:16,770 we do today today.

85 00:02:16,779 –> 00:02:18,149 We prove with four vectors

86 00:02:18,160 –> 00:02:18,869 in FN.

87 00:02:19,649 –> 00:02:20,880 In order to prove Hölder’s

88 00:02:20,889 –> 00:02:22,610 inequality, we first need

89 00:02:22,619 –> 00:02:24,130 another inequality which

90 00:02:24,139 –> 00:02:25,490 is known as Young’s

91 00:02:25,500 –> 00:02:26,440 inequality.

92 00:02:27,110 –> 00:02:28,649 In fact, this one is very

93 00:02:28,660 –> 00:02:29,250 simple.

94 00:02:29,259 –> 00:02:30,589 We just look at positive

95 00:02:30,600 –> 00:02:32,550 numbers A B and

96 00:02:32,559 –> 00:02:33,850 then we conclude that the

97 00:02:33,860 –> 00:02:35,529 product of A B is

98 00:02:35,539 –> 00:02:37,210 always less or equal than

99 00:02:37,220 –> 00:02:38,309 the following sum.

100 00:02:38,470 –> 00:02:39,630 So what you should see here

101 00:02:39,639 –> 00:02:41,160 is that P greater than one

102 00:02:41,169 –> 00:02:42,850 goes in and the Hölder

103 00:02:42,860 –> 00:02:43,610 conjugate.

104 00:02:44,259 –> 00:02:45,940 So as before you can choose

105 00:02:45,949 –> 00:02:47,669 P greater one as you want,

106 00:02:47,690 –> 00:02:49,470 but then P prime is fixed.

107 00:02:50,000 –> 00:02:51,610 For example, for P equals

108 00:02:51,619 –> 00:02:53,570 to two, we have P prime equals

109 00:02:53,580 –> 00:02:54,110 to two.

110 00:02:54,600 –> 00:02:55,910 However, in this case, you

111 00:02:55,919 –> 00:02:57,619 already know that the inequality

112 00:02:57,630 –> 00:02:58,509 here is correct.

113 00:02:59,300 –> 00:03:00,479 So please check that.

114 00:03:00,490 –> 00:03:01,759 But for all other cases,

115 00:03:01,770 –> 00:03:03,380 we have to write down a proof.

116 00:03:04,190 –> 00:03:05,570 What we can use here is that

117 00:03:05,580 –> 00:03:07,210 the common exponential function

118 00:03:07,220 –> 00:03:08,929 is a so called convex

119 00:03:08,940 –> 00:03:09,369 function.

120 00:03:10,059 –> 00:03:11,190 This means that when you

121 00:03:11,199 –> 00:03:12,279 look at the graph,

122 00:03:12,970 –> 00:03:14,389 then you can choose any two

123 00:03:14,399 –> 00:03:14,779 points.

124 00:03:14,789 –> 00:03:15,630 As you want.

125 00:03:15,710 –> 00:03:16,929 The direct connection would

126 00:03:16,940 –> 00:03:18,630 be always above the graph.

127 00:03:19,110 –> 00:03:20,940 This property is called convex

128 00:03:20,949 –> 00:03:22,509 because the red line is the

129 00:03:22,520 –> 00:03:24,100 convex combination of the

130 00:03:24,110 –> 00:03:24,839 two points.

131 00:03:25,440 –> 00:03:26,479 Therefore, we can easily

132 00:03:26,490 –> 00:03:28,029 put that into a formula when

133 00:03:28,039 –> 00:03:29,449 we call the function just

134 00:03:29,460 –> 00:03:29,910 F.

135 00:03:30,360 –> 00:03:31,990 Now by denoting the two points

136 00:03:32,000 –> 00:03:33,669 by X and Y, we can form a

137 00:03:33,679 –> 00:03:35,410 convex combination of X and

138 00:03:35,419 –> 00:03:37,369 Y and put that into the function

139 00:03:37,380 –> 00:03:39,300 F which means we get out

140 00:03:39,309 –> 00:03:40,220 the blue line here.

141 00:03:40,589 –> 00:03:42,339 Please recall a convex

142 00:03:42,350 –> 00:03:43,940 combination is just a special

143 00:03:43,949 –> 00:03:45,179 linear combination where

144 00:03:45,190 –> 00:03:46,940 we only have one lambda which

145 00:03:46,949 –> 00:03:48,029 comes from the interval

146 00:03:48,039 –> 00:03:49,199 0 to 1.

147 00:03:49,350 –> 00:03:50,660 Now with the red line, we

148 00:03:50,669 –> 00:03:52,289 also get a convex combination

149 00:03:52,300 –> 00:03:53,789 with the images, which means

150 00:03:53,800 –> 00:03:55,029 we have here instead of X

151 00:03:55,039 –> 00:03:56,899 and Y, just F of X

152 00:03:56,940 –> 00:03:57,899 and F of Y.

153 00:03:58,309 –> 00:03:59,770 Therefore being always above

154 00:03:59,779 –> 00:04:01,199 the blue graph means we have

155 00:04:01,210 –> 00:04:02,669 fear our inequality

156 00:04:03,410 –> 00:04:04,949 which we now want to use

157 00:04:04,960 –> 00:04:06,229 for some special numbers.

158 00:04:06,960 –> 00:04:08,339 Lambda should be one over

159 00:04:08,350 –> 00:04:10,110 P and then one minus

160 00:04:10,119 –> 00:04:11,509 lambda should be one over

161 00:04:11,520 –> 00:04:12,240 P prime.

162 00:04:12,729 –> 00:04:13,789 Maybe that’s not so clear.

163 00:04:13,800 –> 00:04:15,360 But for X I want to put in

164 00:04:15,369 –> 00:04:16,720 the natural logarithm of

165 00:04:16,730 –> 00:04:18,108 A to the power P

166 00:04:18,459 –> 00:04:19,640 and the similar thing for

167 00:04:19,649 –> 00:04:21,559 Y but now with B and P

168 00:04:21,570 –> 00:04:23,540 prime, of course, this

169 00:04:23,549 –> 00:04:25,059 all fits together because

170 00:04:25,070 –> 00:04:26,329 by applying the logarithm

171 00:04:26,339 –> 00:04:27,779 rules, we can bring this

172 00:04:27,790 –> 00:04:29,559 power in front where it cancels

173 00:04:29,570 –> 00:04:30,779 out. Hence,

174 00:04:30,790 –> 00:04:31,779 on the left hand side, we

175 00:04:31,790 –> 00:04:33,029 have the two logarithms in

176 00:04:33,040 –> 00:04:33,940 the function F.

177 00:04:34,320 –> 00:04:36,089 Now using the next logarithm

178 00:04:36,100 –> 00:04:36,619 rule.

179 00:04:36,630 –> 00:04:38,239 And the fact that the exponential

180 00:04:38,250 –> 00:04:39,820 function is the inverse function

181 00:04:39,829 –> 00:04:41,179 of the logarithm, we have

182 00:04:41,190 –> 00:04:42,859 just A B on the left hand

183 00:04:42,869 –> 00:04:43,290 side.

184 00:04:43,750 –> 00:04:44,980 And now you see the idea

185 00:04:44,989 –> 00:04:46,390 of the whole proof, we want

186 00:04:46,399 –> 00:04:48,140 to show this one and we’re

187 00:04:48,149 –> 00:04:49,380 already finished with the

188 00:04:49,390 –> 00:04:50,179 left part

189 00:04:50,920 –> 00:04:51,940 regarding the right hand

190 00:04:51,950 –> 00:04:53,220 side, we just put in all

191 00:04:53,230 –> 00:04:54,609 the numbers we already know.

192 00:04:55,290 –> 00:04:56,869 Now this looks more complicated

193 00:04:56,880 –> 00:04:58,230 than it really is because

194 00:04:58,239 –> 00:04:59,500 we have again the inverse

195 00:04:59,510 –> 00:05:00,670 function of the logarithm

196 00:05:00,679 –> 00:05:01,029 here.

197 00:05:01,579 –> 00:05:03,170 Hence this all vanishes.

198 00:05:03,179 –> 00:05:04,309 And what remains is what

199 00:05:04,320 –> 00:05:05,309 we wanted to show.

200 00:05:05,799 –> 00:05:07,320 So Young’s inequality is

201 00:05:07,329 –> 00:05:08,829 correct and we can use it

202 00:05:08,839 –> 00:05:10,750 to prove Hölder’s inequality

203 00:05:11,260 –> 00:05:12,410 starting with the proof.

204 00:05:12,420 –> 00:05:13,480 Let’s first consider the

205 00:05:13,489 –> 00:05:14,420 simplest case.

206 00:05:15,079 –> 00:05:16,320 What I mean by that is that

207 00:05:16,329 –> 00:05:17,549 we look what happens when

208 00:05:17,559 –> 00:05:19,239 X is the zero vector or

209 00:05:19,250 –> 00:05:20,579 Y is the zero vector.

210 00:05:21,179 –> 00:05:22,459 But of course, this is not

211 00:05:22,470 –> 00:05:23,899 a problem for us because

212 00:05:23,910 –> 00:05:25,269 by the definition of all

213 00:05:25,279 –> 00:05:26,799 the norms here we have that

214 00:05:26,809 –> 00:05:28,250 the left hand side is zero

215 00:05:28,260 –> 00:05:29,459 and the right hand side is

216 00:05:29,470 –> 00:05:29,850 zero.

217 00:05:29,859 –> 00:05:31,820 So the inequality is fulfilled.

218 00:05:32,480 –> 00:05:33,739 Therefore, for the second

219 00:05:33,750 –> 00:05:35,529 case, we can divide and bring

220 00:05:35,540 –> 00:05:36,660 everything to the left hand

221 00:05:36,670 –> 00:05:37,070 side.

222 00:05:37,700 –> 00:05:38,880 Of course, now we want to

223 00:05:38,890 –> 00:05:40,609 bring this inside the norm

224 00:05:40,619 –> 00:05:42,290 and divide X by its P

225 00:05:42,299 –> 00:05:44,250 norm and Y by its P prime

226 00:05:44,260 –> 00:05:44,640 norm.

227 00:05:45,019 –> 00:05:46,380 However, since I use this

228 00:05:46,390 –> 00:05:47,760 strange notation here, I

229 00:05:47,769 –> 00:05:49,339 bring in now the whole sum

230 00:05:49,350 –> 00:05:51,299 of the one norm here we have

231 00:05:51,309 –> 00:05:52,649 it and we can pull in the

232 00:05:52,660 –> 00:05:54,250 norm into the sum and then

233 00:05:54,260 –> 00:05:55,299 into the absolute value.

234 00:05:55,989 –> 00:05:56,929 And there you should see

235 00:05:56,940 –> 00:05:58,679 now that we have two positive

236 00:05:58,690 –> 00:06:00,149 numbers which we could call

237 00:06:00,160 –> 00:06:01,989 A and B and then apply

238 00:06:02,000 –> 00:06:03,899 Young’s inequality inside

239 00:06:03,910 –> 00:06:04,459 a sum.

240 00:06:04,980 –> 00:06:06,380 So at this point, we get

242 00:06:08,399 –> 00:06:08,700 OK.

243 00:06:08,709 –> 00:06:10,010 The first term is one over

244 00:06:10,019 –> 00:06:11,940 P times XJ

245 00:06:11,950 –> 00:06:13,739 to the power P divided by

246 00:06:13,750 –> 00:06:15,450 the P norm to the power P.

247 00:06:15,950 –> 00:06:17,410 The second term looks similar

248 00:06:17,420 –> 00:06:18,570 and I already distributed

249 00:06:18,579 –> 00:06:20,089 the sum over both parts.

250 00:06:20,739 –> 00:06:22,329 So here we just have P prime

251 00:06:22,339 –> 00:06:23,799 and J instead of X.

252 00:06:24,220 –> 00:06:25,769 However, that’s not so important.

253 00:06:25,779 –> 00:06:26,869 The important thing you should

254 00:06:26,880 –> 00:06:28,279 see is what we have in

255 00:06:28,290 –> 00:06:29,600 enumerator together with

256 00:06:29,609 –> 00:06:31,109 the sum is the same as the

257 00:06:31,119 –> 00:06:32,809 denominator and the

258 00:06:32,820 –> 00:06:34,119 same in the second part.

259 00:06:34,549 –> 00:06:35,760 Hence, the only things that

260 00:06:35,769 –> 00:06:37,640 remain is one over P here

261 00:06:37,649 –> 00:06:39,160 and one over P prime here.

262 00:06:39,630 –> 00:06:40,880 And by the definition of

263 00:06:40,890 –> 00:06:42,470 the Hölder conjugate this

264 00:06:42,480 –> 00:06:43,480 is simply one.

265 00:06:44,000 –> 00:06:45,540 And with this, we have proven

266 00:06:45,549 –> 00:06:47,200 Hölders inequality because

267 00:06:47,209 –> 00:06:48,630 you can bring this one on

268 00:06:48,640 –> 00:06:49,760 the right hand side again,

269 00:06:49,769 –> 00:06:50,440 if you want.

270 00:06:51,130 –> 00:06:52,459 So you see the proof was

271 00:06:52,470 –> 00:06:53,869 not so hard, but we will

272 00:06:53,880 –> 00:06:55,630 need Hölder’s inequality to

273 00:06:55,640 –> 00:06:57,309 prove another inequality.

274 00:06:57,739 –> 00:06:58,839 And this one will be the

275 00:06:58,850 –> 00:07:00,149 so called Minkowski

276 00:07:00,160 –> 00:07:01,010 inequality.

277 00:07:01,320 –> 00:07:02,790 Essentially, it’s just the

278 00:07:02,940 –> 00:07:04,869 triangle inequality for our LP

279 00:07:04,880 –> 00:07:05,470 space.

280 00:07:06,010 –> 00:07:07,220 Therefore, I hope I see you

281 00:07:07,230 –> 00:07:08,070 in the next video.

282 00:07:08,079 –> 00:07:09,579 Thanks for listening and

283 00:07:09,589 –> 00:07:10,410 see you then.

284 00:07:10,420 –> 00:07:11,109 Bye.

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