Solving cyclic inequalities

I understand that in case of symmetric inequalities, you can assume abc a \geq b \geq c without loss of generality because you can replace for example aa with bb but still the inequality remains unchanged.

But in case of cyclic inequalities can we assume a=max(a,b,c) a = max(a,b,c) without loss of generality. If yes please explain the reason of WLOG in this case as well.

Example:- [RMO 2017 P6]

Let x,y,zx,y,z be real numbers, each greater than 11. Prove that x+1y+1+y+1z+1+z+1x+1x1y1+y1z1+z1x1\dfrac{x+1}{y+1}+\dfrac{y+1}{z+1}+\dfrac{z+1}{x+1} \leq \dfrac{x-1}{y-1}+\dfrac{y-1}{z-1}+\dfrac{z-1}{x-1}

The official solution uses a=max(a,b,c) a = max(a,b,c) .

#rmo #inequalities #wlog

Note by Santu Paul
2 years, 3 months ago

No vote yet
1 vote

  Easy Math Editor

This discussion board is a place to discuss our Daily Challenges and the math and science related to those challenges. Explanations are more than just a solution — they should explain the steps and thinking strategies that you used to obtain the solution. Comments should further the discussion of math and science.

When posting on Brilliant:

  • Use the emojis to react to an explanation, whether you're congratulating a job well done , or just really confused .
  • Ask specific questions about the challenge or the steps in somebody's explanation. Well-posed questions can add a lot to the discussion, but posting "I don't understand!" doesn't help anyone.
  • Try to contribute something new to the discussion, whether it is an extension, generalization or other idea related to the challenge.
  • Stay on topic — we're all here to learn more about math and science, not to hear about your favorite get-rich-quick scheme or current world events.

MarkdownAppears as
*italics* or _italics_ italics
**bold** or __bold__ bold

- bulleted
- list

  • bulleted
  • list

1. numbered
2. list

  1. numbered
  2. list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1

paragraph 2

paragraph 1

paragraph 2

[example link]( link
> This is a quote
This is a quote
    # I indented these lines
    # 4 spaces, and now they show
    # up as a code block.

    print "hello world"
# I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
MathAppears as
Remember to wrap math in \( ... \) or \[ ... \] to ensure proper formatting.
2 \times 3 2×3 2 \times 3
2^{34} 234 2^{34}
a_{i-1} ai1 a_{i-1}
\frac{2}{3} 23 \frac{2}{3}
\sqrt{2} 2 \sqrt{2}
\sum_{i=1}^3 i=13 \sum_{i=1}^3
\sin \theta sinθ \sin \theta
\boxed{123} 123 \boxed{123}


Sort by:

Top Newest

Since the inequality (or equality) is cyclic, you can get new solutions by "rotating around" the numbers of a solution.

For example, if (5,2,9) (5,2,9) is a solution, then (2,9,5) (2,9,5) and (9,5,2) (9,5,2) are solutions as well, but (5,9,2) (5,9,2) isn't a solution.

This works because cyclic (in)equalities only use some operation (in your example a+1b+1 \frac {a+1}{b+1} for some a,b a,b ) of all pairs of two "consecutive" variables, but since all "consecutive" variable pairs ((x,y) (x,y) , (y,z) (y,z) and (z,x) (z,x) ) are used in the (in)equality, the order of the variables matters, but the "rotation" doesn't and that's why you can set a=max(a,b,c) a = \text{max} (a,b,c) .

Henry U - 2 years, 3 months ago

Log in to reply

Imagine all 3 (or any number of) variables arranged in a circle. The cyclic (in)equality involves some operation of all pairs of variables that are next to each other in this circle. You can't change the order of the elements in the circle and you also can't make a mirror image, but you can rotate the variables around and WLOG rotate one specific value (in your example the macimum value) so that it becomes a a .

Henry U - 2 years, 3 months ago

Log in to reply

I kind of get the geometric intuition but could you please be a bit more descriptive, especially in the algebraic one. Thank you.

SANTU PAUL - 2 years, 3 months ago

Log in to reply

If a cyclic equality has a solution (x,y,z)1=(a,b,c) (x,y,z)_1 = (a,b,c) , then – since the equality is cyclic – you can get another solution as (x,y,z)2=(b,c,a) (x,y,z)_2 = (b,c,a) . I think you might understand this intuitively, but to prove it algebraically we have to define what exactly a cyclic equality is.

My first thought is the definition

A cyclic equality is an equality f(x,y,z)=0 f(x,y,z) = 0 such that you can substitute (a,b,c)=(g(x,y),g(y,z),g(z,x)) (a,b,c) = (g(x,y),g(y,z),g(z,x)) for some g(r,s) g(r,s) into the equation and get a symmetric equation. In some cases you might have to make multiple substitutions.

Derived from the case of your inequality, we can show that

x+1y+1+y+1z+1+z+1x+1=0 \frac {x+1}{y+1} + \frac {y+1}{z+1} + \frac {z+1}{x+1} = 0

is cyclic (by my definition) because the function g(r,s)=r+1s+1 g(r,s) = \frac {r+1}{s+1} and its corresponding substitution (a,b,c)=(g(x,y),g(y,z),g(z,x)) (a,b,c) = (g(x,y),g(y,z),g(z,x)) gives the symmetric equality

a+b+c=0 a + b + c = 0 .

If we instead had the equality

xyyz+yzzx+zxxy=0 \frac {x-y}{y-z} + \frac {y-z}{z-x} + \frac {z-x}{x-y} = 0

then we would have to use the function g1(r,s)=rs g_1(r,s) = r-s

and get

ab+bc+ca=0 \frac ab + \frac bc + \frac ca = 0

Since this isn't symmetric, we have to use another substitution (d,e,f)=(g2(d,e),g2(e,f),g2(f,d)) (d,e,f) = (g_2(d,e),g_2(e,f),g_2(f,d)) for g2(r,s)=rs g_2(r,s) = \frac rs . This brings us to the symmetric equality

d+e+f=0 d + e + f = 0

Another possibility is to define a cyclic equality as an equality where we can get a new solution from a known one (x,y,z)1=(a,b,c) (x,y,z)_1 = (a,b,c) as (x,y,z)2=(b,c,a) (x,y,z)_2 = (b,c,a) .

Then, this fact you asked about is the defining property and therefore doesn't require any proof.

Which definition do you like the most, or do you have any other ideas?

Henry U - 2 years, 3 months ago

Log in to reply


Problem Loading...

Note Loading...

Set Loading...