Second derivative - wikipedia Second derivative Part of a series of articles about Calculus Fundamental theorem Limits of functions Continuity Mean value theorem Rolle 's theorem Differential Definitions Derivative ( generalizations ) Differential infinitesimal of a function total Concepts Differentiation notation Second derivative Third derivative Change of variables Implicit differentiation Related rates Taylor 's theorem Rules and identities Sum Product Chain Power Quotient Inverse General Leibniz Faà di Bruno 's formula Integral Lists of integrals Definitions Antiderivative Integral ( improper ) Riemann integral Lebesgue integration Contour integration Integration by Parts Discs Cylindrical shells Substitution ( trigonometric ) Partial fractions Order Reduction formulae Series Geometric ( arithmetico - geometric ) Harmonic Alternating Power Binomial Taylor Convergence tests Summand limit ( term test ) Ratio Root Integral Direct comparison Limit comparison Alternating series Cauchy condensation Dirichlet Abel Vector Gradient Divergence Curl Laplacian Directional derivative Identities Theorems Divergence Gradient Green 's Kelvin -- Stokes Stokes Multivariable Formalisms Matrix Tensor Exterior Geometric Definitions Partial derivative Multiple integral Line integral Surface integral Volume integral Jacobian Hessian Specialized Fractional Malliavin Stochastic Variations Glossary of calculus Glossary of calculus The second derivative of a quadratic function is constant. In calculus, the second derivative, or the second order derivative, of a function f is the derivative of the derivative of f. Roughly speaking, the second derivative measures how the rate of change of a quantity is itself changing ; for example, the second derivative of the position of a vehicle with respect to time is the instantaneous acceleration of the vehicle, or the rate at which the velocity of the vehicle is changing with respect to time. In Leibniz notation : a = d v d t = d 2 x d t 2, ( displaystyle mathbf ( a ) = ( \frac ( d mathbf ( v ) ) ( dt ) ) = ( \frac ( d ^ ( 2 ) ( \boldsymbol ( x ) ) ) ( dt ^ ( 2 ) ) ), ) where the last term is the second derivative expression. On the graph of a function, the second derivative corresponds to the curvature or concavity of the graph. The graph of a function with a positive second derivative is upwardly concave, while the graph of a function with a negative second derivative curves in the opposite way. Contents 1 Second derivative power rule 2 Notation 3 Example 4 Relation to the graph 4.1 Concavity 4.2 Inflection points 4.3 Second derivative test 5 Limit 6 Quadratic approximation 7 Eigenvalues and eigenvectors of the second derivative 8 Generalization to higher dimensions 8.1 The Hessian 8.2 The Laplacian 9 See also 10 References 11 Further reading 11.1 Print 11.2 Online books 12 External links Second derivative power rule The power rule for the first derivative, if applied twice, will produce the second derivative power rule as follows : d 2 d x 2 ( x n ) = d d x d d x ( x n ) = d d x ( n x n − 1 ) = n d d x ( x n − 1 ) = n ( n − 1 ) x n − 2. ( displaystyle ( \frac ( d ^ ( 2 ) ) ( dx ^ ( 2 ) ) ) ( x ^ ( n ) ) = ( \frac ( d ) ( dx ) ) ( \frac ( d ) ( dx ) ) ( x ^ ( n ) ) = ( \frac ( d ) ( dx ) ) ( nx ^ ( n - 1 ) ) = n ( \frac ( d ) ( dx ) ) ( x ^ ( n - 1 ) ) = n ( n - 1 ) x ^ ( n - 2 ). ) Notation For more details on this topic, see Notation for differentiation. The second derivative of a function f ( x ) ( displaystyle f ( x ) ) is usually denoted f '' ( x ) ( displaystyle f ' ' ( x ) ). That is : f '' = ( f ′ ) ′ ( displaystyle f ' ' = ( f ' ) ' ) When using Leibniz 's notation for derivatives, the second derivative of a dependent variable y with respect to an independent variable x is written d 2 y d x 2. ( displaystyle ( \frac ( d ^ ( 2 ) y ) ( dx ^ ( 2 ) ) ). ) This notation is derived from the following formula : d 2 y d x 2 = d d x ( d y d x ). ( displaystyle ( \frac ( d ^ ( 2 ) y ) ( dx ^ ( 2 ) ) ) , = , ( \frac ( d ) ( dx ) ) left ( ( \frac ( dy ) ( dx ) ) \right ). ) Example Given the function f ( x ) = x 3, ( displaystyle f ( x ) = x ^ ( 3 ), ) the derivative of f is the function f ′ ( x ) = 3 x 2. ( displaystyle f ' ( x ) = 3x ^ ( 2 ). ) The second derivative of f is the derivative of f ′, namely f '' ( x ) = 6 x. ( displaystyle f ' ' ( x ) = 6x. ) Relation to the graph A plot of f ( x ) = sin ⁡ ( 2 x ) ( displaystyle f ( x ) = sin ( 2x ) ) from − π / 4 ( displaystyle - pi / 4 ) to 5 π / 4 ( displaystyle 5 pi / 4 ). The tangent line is blue where the curve is concave up, green where the curve is concave down, and red at the inflection points ( 0, π ( displaystyle pi ) / 2, and π ( displaystyle pi ) ). Concavity The second derivative of a function f measures the concavity of the graph of f. A function whose second derivative is positive will be concave up ( also referred to as convex ), meaning that the tangent line will lie below the graph of the function. Similarly, a function whose second derivative is negative will be concave down ( also simply called concave ), and its tangent lines will lie above the graph of the function. Inflection points Main article : Inflection point If the second derivative of a function changes sign, the graph of the function will switch from concave down to concave up, or vice versa. A point where this occurs is called an inflection point. Assuming the second derivative is continuous, it must take a value of zero at any inflection point, although not every point where the second derivative is zero is necessarily a point of inflection. Second derivative test Main article : Second derivative test The relation between the second derivative and the graph can be used to test whether a stationary point for a function ( i.e. a point where f ′ ( x ) = 0 ( displaystyle f ' ( x ) = 0 ) ) is a local maximum or a local minimum. Specifically, If f ′ ′ ( x ) < 0 ( displaystyle \f ^ ( prime prime ) ( x ) < 0 ) then f ( displaystyle \f ) has a local maximum at x ( displaystyle \x ). If f ′ ′ ( x ) > 0 ( displaystyle \f ^ ( prime prime ) ( x ) > 0 ) then f ( displaystyle \f ) has a local minimum at x ( displaystyle \x ). If f ′ ′ ( x ) = 0 ( displaystyle \f ^ ( prime prime ) ( x ) = 0 ), the second derivative test says nothing about the point x ( displaystyle \x ), a possible inflection point. The reason the second derivative produces these results can be seen by way of a real - world analogy. Consider a vehicle that at first is moving forward at a great velocity, but with a negative acceleration. Clearly the position of the vehicle at the point where the velocity reaches zero will be the maximum distance from the starting position -- after this time, the velocity will become negative and the vehicle will reverse. The same is true for the minimum, with a vehicle that at first has a very negative velocity but positive acceleration. Limit It is possible to write a single limit for the second derivative : f '' ( x ) = lim h → 0 f ( x + h ) − 2 f ( x ) + f ( x − h ) h 2. ( displaystyle f ' ' ( x ) = lim _ ( h \to 0 ) ( \frac ( f ( x + h ) - 2f ( x ) + f ( x-h ) ) ( h ^ ( 2 ) ) ). ) The limit is called the second symmetric derivative. Note that the second symmetric derivative may exist even when the ( usual ) second derivative does not. The expression on the right can be written as a difference quotient of difference quotients : f ( x + h ) − 2 f ( x ) + f ( x − h ) h 2 = f ( x + h ) − f ( x ) h − f ( x ) − f ( x − h ) h h. ( displaystyle ( \frac ( f ( x + h ) - 2f ( x ) + f ( x-h ) ) ( h ^ ( 2 ) ) ) = ( \frac ( ( \frac ( f ( x + h ) - f ( x ) ) ( h ) ) - ( \frac ( f ( x ) - f ( x-h ) ) ( h ) ) ) ( h ) ). ) This limit can be viewed as a continuous version of the second difference for sequences. Please note that the existence of the above limit does not mean that the function f ( displaystyle f ) has a second derivative. The limit above just gives a possibility for calculating the second derivative but does not provide a definition. As a counterexample look on the sign function sgn ⁡ ( x ) ( displaystyle operatorname ( sgn ) ( x ) ) which is defined through sgn ⁡ ( x ) = ( − 1 if x < 0, 0 if x = 0, 1 if x > 0. ( displaystyle operatorname ( sgn ) ( x ) = ( \begin ( cases ) - 1& ( \text ( if ) ) x < 0, 0& ( \text ( if ) ) x = 0, 1& ( \text ( if ) ) x > 0. end ( cases ) ) ) The sign function is not continuous at zero and therefore the second derivative for x = 0 ( displaystyle x = 0 ) does not exist. But the above limit exists for x = 0 ( displaystyle x = 0 ) : lim h → 0 sgn ⁡ ( 0 + h ) − 2 sgn ⁡ ( 0 ) + sgn ⁡ ( 0 − h ) h 2 = lim h → 0 1 − 2 ⋅ 0 + ( − 1 ) h 2 = lim h → 0 0 h 2 = 0 ( displaystyle ( \begin ( aligned ) lim _ ( h \to 0 ) ( \frac ( operatorname ( sgn ) ( 0 + h ) - 2 operatorname ( sgn ) ( 0 ) + operatorname ( sgn ) ( 0 - h ) ) ( h ^ ( 2 ) ) ) & = lim _ ( h \to 0 ) ( \frac ( 1 - 2 cdot 0 + ( - 1 ) ) ( h ^ ( 2 ) ) ) & = lim _ ( h \to 0 ) ( \frac ( 0 ) ( h ^ ( 2 ) ) ) & = 0 end ( aligned ) ) ) Quadratic approximation Just as the first derivative is related to linear approximations, the second derivative is related to the best quadratic approximation for a function f. This is the quadratic function whose first and second derivatives are the same as those of f at a given point. The formula for the best quadratic approximation to a function f around the point x = a is f ( x ) ≈ f ( a ) + f ′ ( a ) ( x − a ) + 1 2 f '' ( a ) ( x − a ) 2. ( displaystyle f ( x ) approx f ( a ) + f ' ( a ) ( x-a ) + ( \tfrac ( 1 ) ( 2 ) ) f ' ' ( a ) ( x-a ) ^ ( 2 ). ) This quadratic approximation is the second - order Taylor polynomial for the function centered at x = a. Eigenvalues and eigenvectors of the second derivative For many combinations of boundary conditions explicit formulas for eigenvalues and eigenvectors of the second derivative can be obtained. For example, assuming x ∈ ( 0, L ) ( displaystyle x in ( 0, L ) ) and homogeneous Dirichlet boundary conditions, i.e., v ( 0 ) = v ( L ) = 0 ( displaystyle v ( 0 ) = v ( L ) = 0 ), the eigenvalues are λ j = − j 2 π 2 L 2 ( displaystyle lambda _ ( j ) = - ( \frac ( j ^ ( 2 ) pi ^ ( 2 ) ) ( L ^ ( 2 ) ) ) ) and the corresponding eigenvectors ( also called eigenfunctions ) are v j ( x ) = 2 L sin ⁡ ( j π x L ) ( displaystyle v_ ( j ) ( x ) = ( sqrt ( \frac ( 2 ) ( L ) ) ) sin left ( ( \frac ( j pi x ) ( L ) ) \right ) ). Here, v j '' ( x ) = λ j v j ( x ), j = 1,..., ∞. ( displaystyle v ' ' _ ( j ) ( x ) = lambda _ ( j ) v_ ( j ) ( x ), , j = 1, ldots, infty. ) For other well - known cases, see the main article eigenvalues and eigenvectors of the second derivative. Generalization to higher dimensions The Hessian Main article : Hessian matrix The second derivative generalizes to higher dimensions through the notion of second partial derivatives. For a function f : R → R, these include the three second - order partials ∂ 2 f ∂ x 2, ∂ 2 f ∂ y 2, and ∂ 2 f ∂ z 2 ( displaystyle ( \frac ( partial ^ ( 2 ) f ) ( partial x ^ ( 2 ) ) ), ; ( \frac ( partial ^ ( 2 ) f ) ( partial y ^ ( 2 ) ) ), ( \text ( and ) ) ( \frac ( partial ^ ( 2 ) f ) ( partial z ^ ( 2 ) ) ) ) and the mixed partials ∂ 2 f ∂ x ∂ y, ∂ 2 f ∂ x ∂ z, and ∂ 2 f ∂ y ∂ z. ( displaystyle ( \frac ( partial ^ ( 2 ) f ) ( partial x , partial y ) ), ; ( \frac ( partial ^ ( 2 ) f ) ( partial x , partial z ) ), ( \text ( and ) ) ( \frac ( partial ^ ( 2 ) f ) ( partial y , partial z ) ). ) If the function 's image and domain both have a potential, then these fit together into a symmetric matrix known as the Hessian. The eigenvalues of this matrix can be used to implement a multivariable analogue of the second derivative test. ( See also the second partial derivative test. ) The Laplacian Main article : Laplace operator Another common generalization of the second derivative is the Laplacian. This is the differential operator ∇ 2 ( displaystyle \nabla ^ ( 2 ) ) defined by ∇ 2 f = ∂ 2 f ∂ x 2 + ∂ 2 f ∂ y 2 + ∂ 2 f ∂ z 2. ( displaystyle \nabla ^ ( 2 ) f = ( \frac ( partial ^ ( 2 ) f ) ( partial x ^ ( 2 ) ) ) + ( \frac ( partial ^ ( 2 ) f ) ( partial y ^ ( 2 ) ) ) + ( \frac ( partial ^ ( 2 ) f ) ( partial z ^ ( 2 ) ) ). ) The Laplacian of a function is equal to the divergence of the gradient and the trace of the Hessian matrix. See also Chirpyness, second derivative of instantaneous phase References Jump up ^ A. Zygmund ( 2002 ). Trigonometric Series. Cambridge University Press. pp. 22 -- 23. ISBN 978 - 0 - 521 - 89053 - 3. Jump up ^ Thomson, Brian S. ( 1994 ). Symmetric Properties of Real Functions. Marcel Dekker. p. 1. ISBN 0 - 8247 - 9230 - 0. Further reading Print Anton, Howard ; Bivens, Irl ; Davis, Stephen ( February 2, 2005 ), Calculus : Early Transcendentals Single and Multivariable ( 8th ed. ), New York : Wiley, ISBN 978 - 0 - 471 - 47244 - 5 Apostol, Tom M. ( June 1967 ), Calculus, Vol. 1 : One - Variable Calculus with an Introduction to Linear Algebra, 1 ( 2nd ed. ), Wiley, ISBN 978 - 0 - 471 - 00005 - 1 Apostol, Tom M. ( June 1969 ), Calculus, Vol. 2 : Multi-Variable Calculus and Linear Algebra with Applications, 1 ( 2nd ed. ), Wiley, ISBN 978 - 0 - 471 - 00007 - 5 Eves, Howard ( January 2, 1990 ), An Introduction to the History of Mathematics ( 6th ed. ), Brooks Cole, ISBN 978 - 0 - 03 - 029558 - 4 Larson, Ron ; Hostetler, Robert P. ; Edwards, Bruce H. ( February 28, 2006 ), Calculus : Early Transcendental Functions ( 4th ed. ), Houghton Mifflin Company, ISBN 978 - 0 - 618 - 60624 - 5 Spivak, Michael ( September 1994 ), Calculus ( 3rd ed. ), Publish or Perish, ISBN 978 - 0 - 914098 - 89 - 8 Stewart, James ( December 24, 2002 ), Calculus ( 5th ed. ), Brooks Cole, ISBN 978 - 0 - 534 - 39339 - 7 Thompson, Silvanus P. ( September 8, 1998 ), Calculus Made Easy ( Revised, Updated, Expanded ed. ), New York : St. Martin 's Press, ISBN 978 - 0 - 312 - 18548 - 0 Online books Crowell, Benjamin ( 2003 ), Calculus Garrett, Paul ( 2004 ), Notes on First - Year Calculus Hussain, Faraz ( 2006 ), Understanding Calculus Keisler, H. Jerome ( 2000 ), Elementary Calculus : An Approach Using Infinitesimals Mauch, Sean ( 2004 ), Unabridged Version of Sean 's Applied Math Book Sloughter, Dan ( 2000 ), Difference Equations to Differential Equations Strang, Gilbert ( 1991 ), Calculus Stroyan, Keith D. ( 1997 ), A Brief Introduction to Infinitesimal Calculus Wikibooks, Calculus External links Discrete Second Derivative from Unevenly Spaced Points Retrieved from `` https://en.wikipedia.org/w/index.php?title=Second_derivative&oldid=809410480 '' Categories : Mathematical analysis Differential calculus Functions and mappings Linear operators in calculus Talk Contents About Wikipedia Català فارسی Français Bahasa Indonesia Nederlands Português Simple English Suomi Edit links This page was last edited on 8 November 2017, at 22 : 49. About Wikipedia

Question: what does it mean if second derivative is negative
Answer: a function whose second derivative is negative will be concave down (also simply called concave), and its tangent lines will lie above the graph of the function