# First derivative test

## Contents

## Statement

### What the test is for

The **first derivative test** is a partial (i.e., not always conclusive) test used to determine whether a particular critical point in the domain of a function is a point where the function attains a local maximum value, local minimum value, or neither. There are cases where the test is *inconclusive*, which means that we cannot draw any conclusion.

### What the test says: one-sided sign versions

Suppose is a function defined at a point .

Then, we have the following:

Continuity and differentiability assumption | Hypothesis on sign of derivative | Conclusion |
---|---|---|

is left continuous at and differentiable on the immediate left of |
is positive (respectively, nonnegative) for to the immediate left of (i.e., for for sufficiently small ) | has a strict local maximum from the left at , i.e., (respectively, has a local maximum from the left at , i.e., ) for to the immediate left of . |

is left continuous at and differentiable on the immediate left of |
is negative (respectively, nonpositive) for to the immediate left of (i.e., for for sufficiently small ) | has a strict local minimum from the left at , i.e., (respectively, has a local minimum from the left at , i.e., ) for to the immediate left of . |

is right continuous at and differentiable on the immediate right of |
is positive (respectively, nonnegative) for to the immediate right of (i.e., for for sufficiently small ) | has a strict local minimum from the right at , i.e., (respectively, has a local minimum from the right at , i.e., ) for to the immediate right of . |

is right continuous at and differentiable on the immediate right of |
is negative (respectively, nonpositive) for to the immediate right of (i.e., for for sufficiently small ) | has a strict local maximum from the right at , i.e., (respectively, has a local maximum from the right at , i.e., ) for to the immediate right of . |

### What the test says: combined sign versions

Suppose is a function defined around a point (i.e., is defined in an open interval containing ) and is continuous at . We do not care whether is differentiable at ; however, the test makes sense only if is differentiable on the immediate left and immediate right of .

Then, we have the following (we list only the *strict* cases in the table below):

Continuity and differentiability assumption | Sign of the derivative on immediate left of | Sign of on immediate right of | Conclusion about local minimum, local maximum, or neither |
---|---|---|---|

is continuous at and differentiable on the immediate left and immediate right of | positive | negative | strict local maximum |

is continuous at and differentiable on the immediate left and immediate right of | negative | positive | strict local minimum |

is continuous at and differentiable on the immediate left and immediate right of | positive | positive | neither local maximum nor local minimum, because is increasing through the point |

is continuous at and differentiable on the immediate left and immediate right of | negative | negative | neither local maximum nor local minimum, because is decreasing through the point |

If we replace *positive* by *nonnegative* and *negative* by *nonpositive* in the rows corresponding to strict local maximum and strict local minimum, we could potentially lose the *strictness*.

Note that if has ambiguous sign on the immediate left or on the immediate right of , the first derivative test is inconclusive.

### Relation with critical points

The typical goal of the first derivative test is to determine whether a critical point is a point of local maximum or minimum. Hence, the test is typically applied to critical points. *However, when applying the first derivative test, we do not need to check whether the point in question is a critical point. In other words, if the condition for being a point of local maximum or minimum is satisfied, then the point in question is automatically a critical point and this condition need not be checked separately.*

### Succinct version

Here is a shorter version: at a critical point, if the derivative changes sign from negative to positive (as we go from left to right) then that is a point of local minimum. If the derivative changes sign from positive to negative (as we go from left to right) then it is a point of local maximum.

## Related tests

## Notes

### Points of inflection

### Examples illustrating why the test is not always conclusive

The following problems could occur when applying this test:

- The function is not continuous, or not differentiable, at points to the immediate left or immediate right of the critical point.
- The function is differentiable on the immediate left and immediate right of the critical point. However, the derivative does not have a uniform sign on the immediate left or the immediate right, i.e., it is oscillatory in sign at points arbitrarily close to the critical point.

Here is a picture of a function illustrating (2):

The function illustrated in the picture is:

We note the following:

- has a local and absolute minimum at 0: For , we have . This is because and , so that is also positive.
- is continuous at 0: We can see this using the pinching theorem or more directly by noting that as , and is bounded between finite values 1 and 3.
- has ambiguous sign on the immediate right of 0: For , we have . The derivative is . The part is bounded, but the part oscillates between large magnitude positive and negative values as . In particular, does not have constant sign on the immediate right of 0.
- has ambiguous sign on the immediate left of 0: For , we have . The derivative is . The part is boundd, but the part oscillates between large magnitude positive and negative values as . In particular, does not have constant sign on the immediate left of 0.