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In calculus, a branch of mathematics, the third derivative is the rate at which the second derivative, or the rate of change of the rate of change, is changing, used to define aberrancy. The third derivative of a function can be denoted by
Other notations can be used, but the above are the most common.
Let . Then , and . Therefore, the third derivative of f(x) is, in this case,
or, using Leibniz notation,
Now for a more general definition. Let be any function of x. Then the third derivative of is given by the following:
The third derivative is the rate at which the second derivative (f''(x)) is changing.
Applications in geometry
In differential geometry, the torsion of a curve — a fundamental property of curves in three dimensions — is computed using third derivatives of coordinate functions (or the position vector) describing the curve.
Applications in physics
where j(t) is the jerk function with respect to time, and r(t) is the position function of the object with respect to time.
When campaigning for a second term in office, U.S. President Richard Nixon announced that the rate of increase of inflation was decreasing, which has been noted as "the first time a sitting president used the third derivative to advance his case for reelection." Since inflation is itself a derivative—the rate at which the purchasing power of money decreases—then the rate of increase of inflation is the derivative of inflation, opposite in sign to the second time derivative of the purchasing power of money. Stating that a function is decreasing is equivalent to stating that its derivative is negative, so Nixon's statement is that the second derivative of inflation is negative, and so the third derivative of purchasing power is positive.
Nixon's statement allowed for the rate of inflation to increase, however, so his statement was not as indicative of stable prices as it sounds.
- Schot, Stephen (November 1978). "Aberrancy: Geometry of the Third Derivative". Mathematics Magazine. 5. 51: 259–275. doi:10.2307/2690245. JSTOR 2690245.
- do Carmo, Manfredo (1976). Differential Geometry of Curves and Surfaces. ISBN 0-13-212589-7.
- Rossi, Hugo (October 1996). "Mathematics Is an Edifice, Not a Toolbox" (PDF). Notices of the American Mathematical Society. 43 (10): 1108. Retrieved 13 November 2012.