Inequality with $(x+y)(y+z)(z+w)(w+x)=1$
Let $x,y,z,w>0$ and such that
$$(x+y)(y+z)(z+w)(w+x)=1.$$
Show that
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le 2.$$
I'm trying to use Holder's inequality
$$(sqrt[3]{xyz}+sqrt[3]{yzw})^3
le (x+y)(y+z)(z+w)$$
$$(sqrt[3]{zwx}+sqrt[3]{wxy})^3le (z+w)(w+x)(x+y)$$
so
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le sqrt[3]{(x+y)(y+z)(z+w)}+sqrt[3]{(z+w)(w+x)(x+y)}.$$
inequality radicals a.m.-g.m.-inequality holder-inequality
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Let $x,y,z,w>0$ and such that
$$(x+y)(y+z)(z+w)(w+x)=1.$$
Show that
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le 2.$$
I'm trying to use Holder's inequality
$$(sqrt[3]{xyz}+sqrt[3]{yzw})^3
le (x+y)(y+z)(z+w)$$
$$(sqrt[3]{zwx}+sqrt[3]{wxy})^3le (z+w)(w+x)(x+y)$$
so
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le sqrt[3]{(x+y)(y+z)(z+w)}+sqrt[3]{(z+w)(w+x)(x+y)}.$$
inequality radicals a.m.-g.m.-inequality holder-inequality
add a comment |
Let $x,y,z,w>0$ and such that
$$(x+y)(y+z)(z+w)(w+x)=1.$$
Show that
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le 2.$$
I'm trying to use Holder's inequality
$$(sqrt[3]{xyz}+sqrt[3]{yzw})^3
le (x+y)(y+z)(z+w)$$
$$(sqrt[3]{zwx}+sqrt[3]{wxy})^3le (z+w)(w+x)(x+y)$$
so
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le sqrt[3]{(x+y)(y+z)(z+w)}+sqrt[3]{(z+w)(w+x)(x+y)}.$$
inequality radicals a.m.-g.m.-inequality holder-inequality
Let $x,y,z,w>0$ and such that
$$(x+y)(y+z)(z+w)(w+x)=1.$$
Show that
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le 2.$$
I'm trying to use Holder's inequality
$$(sqrt[3]{xyz}+sqrt[3]{yzw})^3
le (x+y)(y+z)(z+w)$$
$$(sqrt[3]{zwx}+sqrt[3]{wxy})^3le (z+w)(w+x)(x+y)$$
so
$$sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}le sqrt[3]{(x+y)(y+z)(z+w)}+sqrt[3]{(z+w)(w+x)(x+y)}.$$
inequality radicals a.m.-g.m.-inequality holder-inequality
inequality radicals a.m.-g.m.-inequality holder-inequality
edited Dec 29 '18 at 5:23
Michael Rozenberg
97.4k1589188
97.4k1589188
asked Dec 29 '18 at 1:19
inequalityinequality
701520
701520
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2 Answers
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Since $$prod_{cyc}(x+y)-sum_{cyc}xsum_{cyc}xyz=(xz-yw)^2geq0,$$ by Holder we obtain:
$$2=2left(prod_{cyc}(x+y)right)^{frac{1}{4}}geq2left(sum_{cyc}xsum_{cyc}xyzright)^{frac{1}{4}}=$$
$$=sqrt[4]{left(4^2sum_{cyc}xyzright)sum_{cyc}x}geqsqrt[4]{left(sum_{cyc}sqrt[3]{xyz}right)^3sum_{cyc}x}.$$
Thus, it's enough to prove that
$$sum_{cyc}xgeqsum_{cyc}sqrt[3]{xyz},$$ which is true by AM-GM:
$$sum_{cyc}x=frac{1}{3}sum_{cyc}(x+y+z)geqfrac{1}{3}sum_{cyc}3sqrt[3]{xyz}=sum_{cyc}sqrt[3]{xyz}.$$
Done!
I used Holder for two sequences:
$$16sum_{cyc}xyz=left(sum_{cyc}1right)^2sum_{cyc}xyzgeqleft(sum_{cyc}sqrt[3]{1^2xyz}right)^3=left(sum_{cyc}sqrt[3]{xyz}right)^3$$
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
1
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
1
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
add a comment |
This does not seem the best, but using only Holder's inequality, we can get
$$begin{eqnarray}
S^{12}&le& (x+y+x+y)(y+y+x+x)(y+y+x+x)(z+y+z+y)\&&(y+z+z+y)(y+z+z+y)(z+w+z+w)(z+z+w+w)\&&(z+z+w+w)(x+w+x+w)(x+w+w+x)(x+w+w+x) = 2^{12}left(prod_{text{cyc}}(x+y)right)^3 =2^{12},
end{eqnarray}$$for $S= sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}.$
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
add a comment |
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2 Answers
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active
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2 Answers
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active
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Since $$prod_{cyc}(x+y)-sum_{cyc}xsum_{cyc}xyz=(xz-yw)^2geq0,$$ by Holder we obtain:
$$2=2left(prod_{cyc}(x+y)right)^{frac{1}{4}}geq2left(sum_{cyc}xsum_{cyc}xyzright)^{frac{1}{4}}=$$
$$=sqrt[4]{left(4^2sum_{cyc}xyzright)sum_{cyc}x}geqsqrt[4]{left(sum_{cyc}sqrt[3]{xyz}right)^3sum_{cyc}x}.$$
Thus, it's enough to prove that
$$sum_{cyc}xgeqsum_{cyc}sqrt[3]{xyz},$$ which is true by AM-GM:
$$sum_{cyc}x=frac{1}{3}sum_{cyc}(x+y+z)geqfrac{1}{3}sum_{cyc}3sqrt[3]{xyz}=sum_{cyc}sqrt[3]{xyz}.$$
Done!
I used Holder for two sequences:
$$16sum_{cyc}xyz=left(sum_{cyc}1right)^2sum_{cyc}xyzgeqleft(sum_{cyc}sqrt[3]{1^2xyz}right)^3=left(sum_{cyc}sqrt[3]{xyz}right)^3$$
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
1
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
1
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
add a comment |
Since $$prod_{cyc}(x+y)-sum_{cyc}xsum_{cyc}xyz=(xz-yw)^2geq0,$$ by Holder we obtain:
$$2=2left(prod_{cyc}(x+y)right)^{frac{1}{4}}geq2left(sum_{cyc}xsum_{cyc}xyzright)^{frac{1}{4}}=$$
$$=sqrt[4]{left(4^2sum_{cyc}xyzright)sum_{cyc}x}geqsqrt[4]{left(sum_{cyc}sqrt[3]{xyz}right)^3sum_{cyc}x}.$$
Thus, it's enough to prove that
$$sum_{cyc}xgeqsum_{cyc}sqrt[3]{xyz},$$ which is true by AM-GM:
$$sum_{cyc}x=frac{1}{3}sum_{cyc}(x+y+z)geqfrac{1}{3}sum_{cyc}3sqrt[3]{xyz}=sum_{cyc}sqrt[3]{xyz}.$$
Done!
I used Holder for two sequences:
$$16sum_{cyc}xyz=left(sum_{cyc}1right)^2sum_{cyc}xyzgeqleft(sum_{cyc}sqrt[3]{1^2xyz}right)^3=left(sum_{cyc}sqrt[3]{xyz}right)^3$$
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
1
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
1
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
add a comment |
Since $$prod_{cyc}(x+y)-sum_{cyc}xsum_{cyc}xyz=(xz-yw)^2geq0,$$ by Holder we obtain:
$$2=2left(prod_{cyc}(x+y)right)^{frac{1}{4}}geq2left(sum_{cyc}xsum_{cyc}xyzright)^{frac{1}{4}}=$$
$$=sqrt[4]{left(4^2sum_{cyc}xyzright)sum_{cyc}x}geqsqrt[4]{left(sum_{cyc}sqrt[3]{xyz}right)^3sum_{cyc}x}.$$
Thus, it's enough to prove that
$$sum_{cyc}xgeqsum_{cyc}sqrt[3]{xyz},$$ which is true by AM-GM:
$$sum_{cyc}x=frac{1}{3}sum_{cyc}(x+y+z)geqfrac{1}{3}sum_{cyc}3sqrt[3]{xyz}=sum_{cyc}sqrt[3]{xyz}.$$
Done!
I used Holder for two sequences:
$$16sum_{cyc}xyz=left(sum_{cyc}1right)^2sum_{cyc}xyzgeqleft(sum_{cyc}sqrt[3]{1^2xyz}right)^3=left(sum_{cyc}sqrt[3]{xyz}right)^3$$
Since $$prod_{cyc}(x+y)-sum_{cyc}xsum_{cyc}xyz=(xz-yw)^2geq0,$$ by Holder we obtain:
$$2=2left(prod_{cyc}(x+y)right)^{frac{1}{4}}geq2left(sum_{cyc}xsum_{cyc}xyzright)^{frac{1}{4}}=$$
$$=sqrt[4]{left(4^2sum_{cyc}xyzright)sum_{cyc}x}geqsqrt[4]{left(sum_{cyc}sqrt[3]{xyz}right)^3sum_{cyc}x}.$$
Thus, it's enough to prove that
$$sum_{cyc}xgeqsum_{cyc}sqrt[3]{xyz},$$ which is true by AM-GM:
$$sum_{cyc}x=frac{1}{3}sum_{cyc}(x+y+z)geqfrac{1}{3}sum_{cyc}3sqrt[3]{xyz}=sum_{cyc}sqrt[3]{xyz}.$$
Done!
I used Holder for two sequences:
$$16sum_{cyc}xyz=left(sum_{cyc}1right)^2sum_{cyc}xyzgeqleft(sum_{cyc}sqrt[3]{1^2xyz}right)^3=left(sum_{cyc}sqrt[3]{xyz}right)^3$$
edited Dec 29 '18 at 6:45
answered Dec 29 '18 at 5:00
Michael RozenbergMichael Rozenberg
97.4k1589188
97.4k1589188
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
1
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
1
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
add a comment |
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
1
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
1
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
Can you clarify the Holder part?
– Pratyush Sarkar
Dec 29 '18 at 5:51
1
1
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
@Pratyush Sarkar I added something. See now.
– Michael Rozenberg
Dec 29 '18 at 6:46
1
1
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
Ah, thanks that helps a lot. Nice concise answer.
– Pratyush Sarkar
Dec 29 '18 at 6:56
add a comment |
This does not seem the best, but using only Holder's inequality, we can get
$$begin{eqnarray}
S^{12}&le& (x+y+x+y)(y+y+x+x)(y+y+x+x)(z+y+z+y)\&&(y+z+z+y)(y+z+z+y)(z+w+z+w)(z+z+w+w)\&&(z+z+w+w)(x+w+x+w)(x+w+w+x)(x+w+w+x) = 2^{12}left(prod_{text{cyc}}(x+y)right)^3 =2^{12},
end{eqnarray}$$for $S= sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}.$
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
add a comment |
This does not seem the best, but using only Holder's inequality, we can get
$$begin{eqnarray}
S^{12}&le& (x+y+x+y)(y+y+x+x)(y+y+x+x)(z+y+z+y)\&&(y+z+z+y)(y+z+z+y)(z+w+z+w)(z+z+w+w)\&&(z+z+w+w)(x+w+x+w)(x+w+w+x)(x+w+w+x) = 2^{12}left(prod_{text{cyc}}(x+y)right)^3 =2^{12},
end{eqnarray}$$for $S= sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}.$
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
add a comment |
This does not seem the best, but using only Holder's inequality, we can get
$$begin{eqnarray}
S^{12}&le& (x+y+x+y)(y+y+x+x)(y+y+x+x)(z+y+z+y)\&&(y+z+z+y)(y+z+z+y)(z+w+z+w)(z+z+w+w)\&&(z+z+w+w)(x+w+x+w)(x+w+w+x)(x+w+w+x) = 2^{12}left(prod_{text{cyc}}(x+y)right)^3 =2^{12},
end{eqnarray}$$for $S= sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}.$
This does not seem the best, but using only Holder's inequality, we can get
$$begin{eqnarray}
S^{12}&le& (x+y+x+y)(y+y+x+x)(y+y+x+x)(z+y+z+y)\&&(y+z+z+y)(y+z+z+y)(z+w+z+w)(z+z+w+w)\&&(z+z+w+w)(x+w+x+w)(x+w+w+x)(x+w+w+x) = 2^{12}left(prod_{text{cyc}}(x+y)right)^3 =2^{12},
end{eqnarray}$$for $S= sqrt[3]{xyz}+sqrt[3]{yzw}+sqrt[3]{zwx}+sqrt[3]{wxy}.$
answered Dec 30 '18 at 8:59
SongSong
6,640319
6,640319
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
add a comment |
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
Very nice solution! +1
– Michael Rozenberg
Dec 30 '18 at 9:53
add a comment |
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