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DEEPCHECKS GLOSSARY

# Convex Optimization

## What is Convex Optimization?

Convex optimization is a branch of optimization that works on minimizing a convex objective function subject to convex constraints. Optimization issues are studied in this context when the objective function and feasible set are both convex.

Convex optimization solution is crucial due to the many useful qualities that make it straightforward to solve and study. For instance, in the case of convex optimization problems, the optimal solution is guaranteed to exist in the form of a global minimum. Moreover, numerous effective methods exist for addressing convex optimization problems, allowing their use on problems of enormous size that include a great deal of freedom and restriction.

## Convex Functions

A convex function is a function whose graph is always curved upwards, which means that the line segment connecting any two points on the graph is always above or on the graph itself. In other terms, a function f(x) is convex if and only if:

• f (λx + (1 – λ)y) ≤ λ f (x) + (1 – λ) f(y)

for any x and y in f’s domain and any λ in the interval [0,1]. The convexity requirement is the defining feature of convex functions because of this inequality.

Convex functions are helpful in optimization and other fields of mathematics due to a variety of key features. For example, they are always continuous and have a unique global minimum, implying that convex function optimization issues are often simple to solve. Moreover, the first and second derivatives of a convex function are always well-behaved, making it simple to study the function’s behavior and perform optimization procedures such as gradient descent.

Linear, quadratic, and exponential functions are examples of convex functions. Many loss functions and regularization terms in machine learning are also convex, making them well-suited for optimization issues involving big datasets and sophisticated models.

Convex functions are a fundamental idea in mathematics with many applications in optimization, machine learning, and other fields of science and engineering.

## Non-Convex Functions

A non-convex function has a graph that is not necessarily curved upwards, which means that the line segment connecting any two points on the graph may fall below the graph itself. In other terms, a function f(x) is non-convex if x and y exist in f’s domain and λ in the interval (0,1) such that:

• f (λx + (1-λ)y) > λ f (x) + (1-λ) f (y)

This inequality is the defining feature of a non-convex function since it violates the convexity constraint.

• Non-convex functions have a variety of features that make optimization difficult.

Non-convex functions, unlike convex functions, may have many local minima, implying that optimization techniques may converge to a poor solution rather than the global minimum.

Moreover, the derivatives of non-convex functions may be discontinuous or ill-behaved, making optimization procedures like gradient descent difficult to calculate.

Non-convex functions include polynomial functions with more than two degrees, trigonometric functions, and numerous functions used in machine learning, such as the ReLU activation function. Convex and non-convex functions are nevertheless commonly employed in optimization and machine learning despite their difficulties because they may explain complicated interactions between variables and achieve high accuracy on tough problems.

They are a fundamental subject in mathematics, with many applications in optimization, machine learning, and other fields of science and engineering. Its non-convexity, on the other hand, offers substantial hurdles for optimization algorithms and necessitates the use of specific approaches for effective optimization.

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## Convex Optimization Problem

Convex optimization is the study of optimization problems with convex objective functions and constraint sets. Several desired qualities of convex optimization issues include the uniqueness of the optimum solution, global optimality, and efficient techniques for finding the solution.

A convex optimization problem has the following generic form:

• minimize f(x) under the conditions
g i(x) <= 0, i = 1,…, m
h j(x) = 0, j = 1,…, p

– x is the optimization variable,

– f(x) denotes the convex objective function,

– g i(x) denotes convex inequality constraints, and

– h j(x) denotes affine equality constraints.

The convexity of f(x) and the constraint set is the essential characteristic of this issue, which indicates that the objective function and constraints are “bowl-shaped.”

Portfolio optimization, support vector machines, and signal processing are all examples of convex optimization issues. One typical example is linear programming, which is a subset of convex optimization with a linear objective function and constraints.

Interior point techniques, gradient-based approaches, and subgradient methods may all be used to tackle convex optimization issues. Convex optimization is a great tool for tackling a wide variety of optimization issues since these algorithms have shown to be extremely successful and efficient in practice.