# Convex functions

## Convex functions

1. Show, that f(x) = \|x\| is convex on \mathbb{R}^n.

2. Show, that f(x) = c^\top x + b is convex and concave.

3. Show, that f(x) = x^\top Ax, where A\succeq 0 - is convex on \mathbb{R}^n.

4. Show, that f(A) = \lambda_{max}(A) - is convex, if A \in S^n.

5. Prove, that -\log\det X is convex on X \in S^n_{++}.

6. Show, that f(x) is convex, using first and second order criteria, if f(x) = \sum\limits_{i=1}^n x_i^4.

7. Find the set of x \in \mathbb{R}^n, where the function f(x) = \dfrac{-1}{2(1 + x^\top x)} is convex, strictly convex, strongly convex?

8. Find the values of a,b,c, where f(x,y,z) = x^2 + 2axy + by^2 + cz^2 is convex, strictly convex, strongly convex?

9. Выпуклы ли следующие функции: f(x) = e^x - 1, \; x \in \mathbb{R};\;\;\; f(x_1, x_2) = x_1x_2, \; x \in \mathbb{R}^2_{++};\;\;\; f(x_1, x_2) = 1/(x_1x_2), \; x \in \mathbb{R}^2_{++}?

10. Докажите, что множество S = \left\{ x \in \mathbb{R}^n \mid \prod\limits_{i=1}^n x_i \geq 1 \right\} выпукло.

11. Prove, that function f(X) = \mathbf{tr}(X^{-1}), X \in S^n_{++} is convex, while g(X) = (\det X)^{1/n}, X \in S^n_{++} is concave.

12. Kullback–Leibler divergence between p,q \in \mathbb{R}^n_{++} is:

D(p,q) = \sum\limits_{i=1}^n (p_i \log(p_i/q_i) - p_i + q_i)

Prove, that D(p,q) \geq 0 \forall p,q \in \mathbb{R}^n_{++} and D(p,q) = 0 \leftrightarrow p = q

Hint: D(p,q) = f(p) - f(q) - \nabla f(q)^\top (p-q), \;\;\;\; f(p) = \sum\limits_{i=1}^n p_i \log p_i

13. Let x be a real variable with the values a_1 < a_2 < \ldots < a_n with probabilities \mathbb{P}(x = a_i) = p_i. Derive the convexity or concavity of the following functions from p on the set of \left\{p \mid \sum\limits_{i=1}^n p_i = 1, p_i \ge 0 \right\}

• \mathbb{E}x
• \mathbb{P}\{x \ge \alpha\}
• \mathbb{P}\{\alpha \le x \le \beta\}
• \sum\limits_{i=1}^n p_i \log p_i​
• \mathbb{V}x = \mathbb{E}(x - \mathbb{E}x)^2
• \mathbf{quartile}(x) = {\operatorname{inf}}\left\{ \beta \mid \mathbb{P}\{x \le \beta\} \ge 0.25 \right\}
14. Определения выпуклости и сильной выпуклости. Критерии выпуклости и сильной выпуклости первого и второго порядков

15. Геометрическая интерпретация выпуклости и сильной выпуклости. (подпирание прямой и параболой)

16. Приведите различные три операции, сохраняющие выпуклость функции.

17. Доказать, что для a,b \ge 0; \;\;\; \theta \in [0,1]

• - \log \left( \dfrac{a+b}{2}\right) \le -\dfrac{\log a + \log b}{2}
• a^\theta b^{1-\theta} \le \theta a + (1 - \theta)b
• Hölder’s inequality: \sum\limits_{i=1}^n x_i y_i \le \left( \sum\limits_{i=1}^n \vert x_i\vert ^p\right)^{1/p} \left( \sum\limits_{i=1}^n \vert y_i\vert^p\right)^{1/p}. For p >1, \;\; \dfrac{1}{p} + \dfrac{1}{q} = 1.

For x, y \in \mathbb{R}^n

18. Доказать, что матричная норма f(X) = \|X\|_2 = \sup\limits_{y \in \mathbb{R}^n} \dfrac{\|Xy\|_2}{\|y\|_2} выпукла.

19. Доказать, что:

• если f(x) - выпукла, то \exp f(x) также выпукла.
• если f(x) - выпукла, то g(x)^p выпукла для p \ge 1, f(x) \ge 0.
• если f(x) - вогнута, то 1/f(x) выпукла для f(x) > 0.
20. Выпукла ли функция f(X, y) = y^T X^{-1}y на множестве \mathbf{dom} f = \{X, y \mid X + X^T \succeq 0\} ? Известно, что эта функция выпукла, если X - симметричная матрица (упражнение - доказать). Докажите выпуклость или приведите простой контрпример.

21. Пусть функция h(x) - выпуклая на \mathbb{R} неубывающая функция, кроме того: h(x) = 0 при x \le 0. Докажите, что функция h\left(\|x\|_2\right) выпукла на \mathbb{R}^n.

22. Is the function returning the arithmetic mean of vector coordinates is a convex one: a(x) = \frac{1}{n}\sum\limits_{i=1}^n x_i, what about geometric mean: g(x) = \prod\limits_{i=1}^n \left(x_i \right)^{1/n}?

23. Show, that the following function is convex on the set of all positive denominators

f(x) = \dfrac{1}{x_1 - \dfrac{1}{x_2 - \dfrac{1}{x_3 - \dfrac{1}{\ldots}}}}, x \in \mathbb{R}^n

24. Влияют ли линейные члены квадратичной функции на ее выпуклость? Сильную выпуклость?

25. Пусть f(x) : \mathbb{R}^n \to \mathbb{R} такова, что \forall x,y \to f\left( \dfrac{x+y}{2}\right) \leq \dfrac{1}{2}(f(x)+f(y)). Является ли такая функция выпуклой?

26. Find the set, on which the function f(x,y) = e^{xy} will be convex.

27. Study the following function of two variables f(x,y) = e^{xy}.

1. Is this function convex?
2. Prove, that this function will be convex on the line x = y.
3. Find another set in \mathbb{R}^2, on which this function will be convex.
28. Is f(x) = -x \ln x - (1-x) \ln (1-x) convex?

29. Prove, that adding \lambda \|x\|_2^2 to any convex function g(x) ensures strong convexity of a resulting function f(x) = g(x) + \lambda \|x\|_2^2. Find the constant of the strong convexity \mu.

30. Prove, that function

f(x) = \log\left( \sum\limits_{i=1}^n e^{x_i}\right)

is convex using any differential criterion.

31. Prove, that a function f is strongly convex with parameter \mu if and only if the function x \mapsto f(x)- \frac{\mu}{2} \|x\|^{2} is convex.

32. Give an example of a function, that satisfies Polyak Lojasiewicz condition, but doesn’t have convexity property.

33. Prove, that if g(x) - convex function, then f(x) = g(x) + \dfrac{\lambda}{2}\|x\|^2_2 will be strongly convex function.

34. Find then f(x) = x^T A x is strongly convex and find strong convexity constant.

35. Let f: \mathbb{R}^n \to \mathbb{R} be the following function: f(x) = \sum\limits_{i=1}^k x_{\lfloor i \rfloor}, where 1 \leq k \leq n, while the symbol x_{\lfloor i \rfloor} stands for the i-th component of sorted (x_{\lfloor 1 \rfloor} - maximum component of x and x_{\lfloor n \rfloor} - minimum component of x) vector of x. Show, that f is a convex function.

36. Consider the function f(x) = x^d, where x \in \mathbb{R}_{+}. Fill the following table with ✅ or ❎. Explain your answers

d Convex Concave Strictly Convex \mu-strongly convex
-2, x \in \mathbb{R}_{++}
-1, x \in \mathbb{R}_{++}
0
0.5
1
\in (1; 2)
2
> 2
37. Prove that the entropy function, defined as

f(x) = -\sum_{i=1}^n x_i \log(x_i),

with \text{dom}(f) = \{x \in \R^n_{++} : \sum_{i=1}^n x_i = 1\}, is strictly concave.

38. Show, that the function f: \mathbb{R}^n_{++} \to \mathbb{R} is convex if f(x) = - \prod\limits_{i=1}^n x_i^{\alpha_i} if \mathbf{1}^T \alpha = 1, \alpha \succeq 0.

39. Show that the maximum of a convex function f over the polyhedron P = \text{conv}\{v_1, \ldots, v_k\} is achieved at one of its vertices, i.e.,

\sup_{x \in P} f(x) = \max_{i=1, \ldots, k} f(v_i).

A stronger statement is: the maximum of a convex function over a closed bounded convex set is achieved at an extreme point, i.e., a point in the set that is not a convex combination of any other points in the set. (you do not have to prove it). Hint: Assume the statement is false, and use Jensen’s inequality.

40. Show, that the two definitions of \mu-strongly convex functions are equivalent:

1. f(x) is \mu-strongly convex \iff for any x_1, x_2 \in S and 0 \le \lambda \le 1 for some \mu > 0:

f(\lambda x_1 + (1 - \lambda)x_2) \le \lambda f(x_1) + (1 - \lambda)f(x_2) - \mu \lambda (1 - \lambda)\|x_1 - x_2\|^2

2. f(x) is \mu-strongly convex \iff if there exists \mu>0 such that the function f(x) - \dfrac{\mu}{2}\Vert x\Vert^2 is convex.