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Bounded sequence

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Bounded sequence

In mathematics, a function f defined on some set X with real or complex values is called bounded, if the set of its values is bounded. In other words, there exists a real number M such that

|f(x)|\le M

for all x in X.

Sometimes, if f(x) ≤ A for all x in X, then the function is said to be bounded above by A. On the other hand, if f(x) ≥ B for all x in X, then the function is said to be bounded below by B.

The concept should not be confused with that of a bounded operator.

An important special case is a bounded sequence, where X is taken to be the set N of natural numbers. Thus a sequence f = (a0, a1, a2, ...) is bounded if there exists a real number M such that

|a_n|\le M

for every natural number n. The set of all bounded sequences, equipped with a vector space structure, forms a sequence space.

This definition can be extended to functions taking values in a metric space Y. Such a function f defined on some set X is called bounded if for some a in Y there exists a real number M such that its distance function d ("distance") is less than M, i.e.

d(f(x), a)\le M

for all x in X.

If this is the case, there is also such an M for each other a, by the triangle inequality.

Examples

  • The function f : RR defined by f(x) = sin(x) is bounded. The sine function is no longer bounded if it is defined over the set of all complex numbers.
  • The function
f(x)=\frac{1}{x^2-1}
defined for all real x except for −1 and 1 is unbounded. As x gets closer to −1 or to 1, the values of this function get larger and larger in magnitude. This function can be made bounded if one considers its domain to be, for example, [2, ∞) or (−∞, −2].
  • The function
f(x)=\frac{1}{x^2+1}
defined for all real x is bounded.
  • Every continuous function f : [0, 1] → R is bounded. This is really a special case of a more general fact: Every continuous function from a compact space into a metric space is bounded.
  • The function f which takes the value 0 for x rational number and 1 for x irrational number is bounded. Thus, a function does not need to be "nice" in order to be bounded. The set of all bounded functions defined on [0, 1] is much bigger than the set of continuous functions on that interval.
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