équivalence

Équivalence is a fundamental concept in logic, mathematics, and other fields, signifying that two statements or expressions have the same meaning or truth value. In formal logic, two propositions are considered equivalent if they are true under precisely the same conditions. This means that if one proposition is true, the other must also be true, and if one is false, the other must also be false.

Symbolically, equivalence is often represented by a double-headed arrow ($\leftrightarrow$) or by the equality sign (=). For example, the statement "P if and only if Q" is an expression of logical equivalence between propositions P and Q. This biconditional statement is true when P and Q have the same truth value, and false otherwise.

In mathematics, equivalence can refer to various types of relationships. For instance, in set theory, two sets are equivalent if there exists a bijection between them, meaning they have the same cardinality. In number theory, two integers can be equivalent modulo some integer $n$ if their difference is divisible by $n$. This concept is crucial for understanding modular arithmetic.

The notion of equivalence is also present in computer science, where two algorithms might be considered equivalent if they produce the same output for the same input, or if they have the same time complexity. In essence, equivalence establishes a relationship of sameness or substitutability between different entities based on specific criteria. It allows for simplification, abstraction, and the transfer of properties from one entity to another.