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A real function f is continuous at a standard real number x if for every hyperreal x' infinitely close to x, the value f(x' ) is also infinitely close to f(x). This captures Cauchy 's definition of continuity as presented in his 1821 textbook Cours d'Analyse , p.
respectively. If these limits exist at p and are equal there, then this can be referred to as the limit of f(x) at p. [7] If the one-sided limits exist at p, but are unequal, then there is no limit at p (i.e., the limit at p does not exist). If either one-sided limit does not exist at p, then the limit at p also does not exist.
This is known as the squeeze theorem. [ 1 ] [ 2 ] This applies even in the cases that f ( x ) and g ( x ) take on different values at c , or are discontinuous at c . Polynomials and functions of the form x a
In other words, since the two one-sided limits exist and are equal, the limit of () as approaches exists and is equal to this same value. If the actual value of f ( x 0 ) {\displaystyle f\left(x_{0}\right)} is not equal to L , {\displaystyle L,} then x 0 {\displaystyle x_{0}} is called a removable discontinuity .
By the intermediate value theorem, every continuous function on a real interval is a Darboux function. Darboux's contribution was to show that there are discontinuous Darboux functions. Every discontinuity of a Darboux function is essential, that is, at any point of discontinuity, at least one of the left hand and right hand limits does not exist.
Limits can be difficult to compute. There exist limit expressions whose modulus of convergence is undecidable. In recursion theory, the limit lemma proves that it is possible to encode undecidable problems using limits. [14] There are several theorems or tests that indicate whether the limit exists. These are known as convergence tests.
In this case, a single limit does not exist because the one-sided limits, L − and L +, exist and are finite, but are not equal: since, L − ≠ L +, the limit L does not exist. Then, x 0 is called a jump discontinuity , step discontinuity , or discontinuity of the first kind .
Blumberg theorem – even if a real function : is nowhere continuous, there is a dense subset of such that the restriction of to is continuous. Thomae's function (also known as the popcorn function) – a function that is continuous at all irrational numbers and discontinuous at all rational numbers.