Surjective

Definition

A function is called surjective (or onto) if every element in the codomain is mapped to by at least one element in the domain. This means that the range of the function covers the entire codomain, ensuring that there are no 'gaps' in the outputs. Surjectivity is important for understanding how functions behave and for determining whether an inverse function can exist.

5 Must Know Facts For Your Next Test

1. For a function to be surjective, every possible output in the codomain must have at least one corresponding input from the domain.
2. If a function is surjective, it guarantees that no element in the codomain is left unmatched, making it a key property for certain mathematical proofs.
3. Surjectivity can be visually represented on a graph where all points in the codomain are reached by arrows from points in the domain.
4. A common example of a surjective function is f(x) = x^3, which maps all real numbers to all real numbers, covering every possible output.
5. To check if a function is surjective, you can often look for an element in the codomain and determine if there exists a corresponding element in the domain that produces it.

Review Questions

• How can you determine if a given function is surjective based on its mapping?
  • To determine if a function is surjective, you need to examine whether every element in the codomain has at least one corresponding element in the domain. This involves checking that for every possible output value, you can find an input value that produces it. If there are outputs without inputs leading to them, then the function is not surjective.
• Compare and contrast surjective and injective functions with examples.
  • Surjective functions cover all elements in their codomain, while injective functions ensure that distinct inputs map to distinct outputs. For example, f(x) = 2x is injective because different x-values yield different outputs, but it isn't surjective if we consider the codomain to be all real numbers since there are no negative outputs. Conversely, f(x) = x^3 is surjective since it reaches every real number as an output, demonstrating a key distinction between how these functions operate.
• Evaluate why surjectivity is important when considering inverse functions and provide an example.
  • Surjectivity is crucial when dealing with inverse functions because only surjective functions can have an inverse that also qualifies as a function. If a function isn't surjective, some elements of the codomain will not correspond to any input from the domain, making it impossible to define an inverse for those elements. For instance, the function f(x) = e^x is surjective when considering its codomain as positive real numbers; hence it has an inverse, ln(x), which returns to its original input values effectively.

Related terms

Injective:

A function is injective (or one-to-one) if different elements in the domain map to different elements in the codomain.

Bijective: A function is bijective if it is both injective and surjective, meaning that every element in the domain maps to a unique element in the codomain and vice versa.

Codomain: The codomain of a function is the set of possible outputs, which can be larger than or equal to the range of the function.