# What is Convex: Definition and 302 Discussions

In geometry, a subset of a Euclidean space, or more generally an affine space over the reals, is convex if, given any two points, it contains the whole line segment that joins them. Equivalently, a convex set or a convex region is a subset that intersects every line into a single line segment (possibly empty).
For example, a solid cube is a convex set, but anything that is hollow or has an indent, for example, a crescent shape, is not convex.
The boundary of a convex set is always a convex curve. The intersection of all the convex sets that contain a given subset A of Euclidean space is called the convex hull of A. It is the smallest convex set containing A.
A convex function is a real-valued function defined on an interval with the property that its epigraph (the set of points on or above the graph of the function) is a convex set. Convex minimization is a subfield of optimization that studies the problem of minimizing convex functions over convex sets. The branch of mathematics devoted to the study of properties of convex sets and convex functions is called convex analysis.
The notion of a convex set can be generalized as described below.

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21. ### MHB Is Every Convex Function Differentiable at Most Points?

Here is this week's POTW: ----- Suppose $f : (a,b) \to \Bbb R$ is a convex function. Show that $f$ is differentiable at all but countably many points and the derivative is nondecreasing. ----- Remember to read the...
22. ### Can a convex reflection be corrected using a concave mirror?

If one took a photograph of a reflection from convex reflective surface. Could one use concave mirror to obtain the original undistorted image?
23. ### Understanding the Impact of Convex Lens Focal Length on Water Temperature

Summary: Hi, I'm doing an assessment for Physics on Optics topics, but I can't really explain how the CV affect the RV CV : Focal length of convex lens RV : Temperature of water after 20 minutes under the sun How I should explain for the temperature difference? 🤔 Please help, Thanks.
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### A What are the equality conditions for proving strict convexity?

Hi PF! Do you know what a strictly convex function is? I understand this notion in the concept of norms, where in the plane I've sketched the ##L_1,L_2,L_\infty## norms, where clearly ##L_1,L_\infty## are not strictly convex and ##L_2## is. Intuitively it would make sense that any...
25. ### Property of compact convex sets of width 1

Homework Statement A strip of width w is a part of the plane bounded by two parallel lines at distance w. The width of a set ##X \subseteq \mathbb{R}^2## is the smallest width of a strip containing ##X##. Prove that a compact convex set of width ##1## contains a segment of length ##1## in every...
26. ### Convex Optimization Without Slater Condition

Hi, initially I am aware of the fact that when slater condition holds, then dual optimum equals primal optimum in convex optimization. But if slater condition does not hold then dual gap exist. When we have nonlinear nonconvex optimization we apply convexification of constraints including...

29. ### Splitting of a convex lens problem

Homework Statement A thin convex lens of focal length 20 cm is split into two parts along the principal axis, now there is a 2mm gap between the top and bottom parts, an object O is placed at 10 cm from the lens, the distance between the two virtual images will be? Homework Equations 1/v - 1/u...
30. ### The use of plano convex lens in a slide projector

Hello My question has two parts: 1) Why is plano convex lens used in slide projector? Why can't we use simple convex lens in it? 2) Why are two plano convex lenses used in slide projector? Why can't we use only one?Thanks!
31. ### MHB Convex Functions: Info on Minima, Reconstructing Original Functions

If you would allow me to ask... if i have two convex functions , and i was to place one inside the other, i.e. convolute them...what could be said in general about the resultant function. what information about the original functions can be taken from the positions of the minima. and is there...
32. ### I Area divided by linear function

The original problem for anyone that can read Chinese: https://zerojudge.tw/ShowProblem?problemid=b221 The problem defines a convex polygon with multiple points located in the first quadrant and the required task is to find a linear function y = ax that can spilt the polygon into two parts each...
33. ### How to Prove Inequality for Convex Sets in R^n?

Homework Statement Let ##C \subset \mathbb{R}^n## a convex set. If ##x \in \mathbb{R}^n## and ##\overline{x} \in C## are points that satisfy ##|x-\overline{x}|=d(x,C)##, proves that ##\langle x-\overline{x},y-\overline{x} \rangle \leq 0## for all ##y \in C##. Homework Equations By definition...
34. ### Proof involving convex function and concave function

Homework Statement [/B] Let X be a vector space over ##\mathbb{R}## and ## f: X \rightarrow \mathbb{R} ## be a convex function and ##g: X \rightarrow \mathbb{R}## be a concave function. Show: The set {##x \in X: f(x) \leq g(x)##} is convex. Homework Equations [/B] If f is convex...
35. ### I Paddle convex and concave sides and forward propulsion

Hello everyone, In stand up paddle boarding, the correct way to use the paddle when moving forward is to have the paddle convex side facing rearward while the paddle pushes water backward. This is explained by the fact that the paddle generates a lift force pointing backward if the convex side...
36. ### Double slits and convex lens interference

Homework Statement Two slits (of width ##a=39 \mu m##) are lighted up with a monocromatic wave of ##\lambda=632,8 nm##. The distance between slits and the screen id ##D=4 m##. The distance between the slits id ##d=195 \mu m##. In front of the slits there are a convergent lens with focal length...
37. ### Photoelectric current and a convex lens

Homework Statement Homework EquationsThe Attempt at a Solution The photoelectric current is directly proportional to the intensity of the light falling on it . It will not depend on the focal length of the lens . When the lens of half the diameter is used , intensity is halved . This...
38. ### Simple microscope and convex lens

When does a convex lens behhave as a simple microscope and what are the conditions for the object and the lens to act if there are any?Is the magnifying power of microscope fixed beyond which it will act as a regular convex lens
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### Understanding Two Convex Lenses: Ray Diagram and Focal Lengths

Homework Statement I'm supposed to make a ray diagram of two convex lenses when light initially hits the first lens at parallel rays. When f(total) goes to infinity, the distance between the two lenses = the sum of the focal lengths of each lens. Homework Equations The Attempt at a Solution...
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### Proof of convex conjugate identity

Homework Statement Prove that the conjugate of ##g(x) = f(Ax + b)## is ## g^*(y) = f^*(A^{-T}y) - b^TA^{-T}y ## where A is nonsingular nXm matrix in R, and b is in ##R^n##. Homework Equations This is from chapter 3 of Boyd's Convex Optimization. 1. The conjugate function is defined as ##...
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### Why Do Objects in Convex Car Mirrors Appear Closer Than They Are?

Homework Statement I have noticed that in cars in India on rearview mirror(convex) it is written that "objects in mirror are closer than they appear". But for convex mirror of focal length 1 metre and object distance 39 metre, the image distance is 39/40 metre . Which tells that image is closer...
42. ### How do you derive the mirror equation with a convex mirror?

Homework Statement Derive the mirror equation for a convex mirror (Si*So=f^2). Do not use a Ray heading toward the center of curvature point (C). Si - distance between the object's image and the focal point. So- distance between the object and the focal point. Do- distance between the mirror...
43. ### Optics - spherical and plane mirror

Homework Statement A convex spherical mirror with a focal length of magnitude 24.0 cm is placed 22.0 cm to the left of a plane mirror. An object 0.300 cm tall is placed midway between the surface of the plane mirror and the vertex of the spherical mirror. The spherical mirror forms multiple...
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### Plano convex lenses and focal length

Does the orientation of a plan convex lens affect it's focal lenght? If I put il like in the first example in the photo and makeep the calculation using the lens maker equation I get f positive What happens if I turn the curved face on the right? Using the lens maker equation now the radius...
45. ### MHB Proving Convexity & Estimating Function $f(x)=(1+x)^n$ for $n\in \mathbb{N}$

Hey! :o I want to show that the function $f(x)=(1+x)^n, x\geq -1$ is for $n\in \mathbb{N}$ convex. So that the function is convex it must hold $f''(x)>0$. The second derivative is $f''(x)=n(n-1)(1+x)^{n-2}$. It holds that $n>0$ and $n-1\geq 0$. We also have that $x\geq -1$. Therefore, we...
46. ### Interpreting: Consider S & T Sets - Are they Convex?

Homework Statement Homework EquationsThe Attempt at a Solution Consider S = {(1,1)} and T = {(0,0)} Clearly, S and T is convex S + T = S and S - T = S So both of them are convex. So answer is (E) But i feel that the answer is too simple...and seems that i wrongly interpreted the question...
47. ### MHB Prove Quadrilateral ABCD Perimeter $\geq (4+2\sqrt 2)S$

A convex quadrilateral ABCD with area $S^2$ , prove the sum of its perimeter and two diagonal lines $\geq (4+2\sqrt 2)S$
48. ### MHB Is Set M1 Convex? A Proof Using Mathematical Induction

Hey! :o We have the function $\displaystyle{y=f(x_1, x_2)=x_1\cdot x_2^2}$ and the set $M_1=\{x\in [0, \infty)^2 \mid f(x_1, x_2)>1\}$. I want to check if the set is convex. Let $x=(x_1, x_2) , y=(y_1, y_2)\in M_1$, then $x_1\cdot x_2^2>1$ and $y_1\cdot y_2^2>1$. We want to show that...
49. ### MHB Proving Convex Set Properties to Showing the Convexity of X-Y

I need help on this problem: If $X$ and $Y$ are convex sets, show that $X-Y = Z = \{x-y \mid x \in X, y \in Y\}$ is also convex. Here are the steps I have gone so far: Let $p \in Z$ such that $p = x_1 - y_1$, and let $q \in Z$ such that $q = x_2 - y_2$. Assume that $r$ lays in the segment...
50. ### Find the Power of a Bi Convex Lens

Homework Statement If the focal length of the bi convex lens is 25 cm then the power of the lens will be? Since the light striking the lens converges at a point behind the lens will the answer come in positive? Homework Equations Power of lens D= 1/f The Attempt at a Solution D= 1/25= +0.04...