In the fall of 1972, 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."
This means that the "jerk (3rd derivative)" of the price level is negative.
Equivalently, the "jerk" of purchasing power is positive.
In other words, the "acceleration" of inflation is negative.
\[I(t)=\alpha^2\dfrac{\mathrm dP(t)}{\mathrm dt}=-\beta^2\dfrac{\mathrm d\Pi(t)}{\mathrm dt}>0\]
\[\dfrac{\mathrm dI(t)}{\mathrm dt}=\alpha^2\dfrac{\mathrm d^2P(t)}{\mathrm dt^2}=-\beta^2\dfrac{\mathrm d^2\Pi(t)}{\mathrm dt^2}>0\]
\[\dfrac{\mathrm d^2I(t)}{\mathrm dt^2}=\alpha^2\dfrac{\mathrm d^3P(t)}{\mathrm dt^3}=-\beta^2\dfrac{\mathrm d^3\Pi(t)}{\mathrm dt^3}<0\]
where, \(I(t)\), \(P(t)\), and \(\Pi(t)\) are inflation (rate), price level (function), and purchasing power of money (or simply value of money) at time \(t\) respectively. And \(\alpha^2\) and \(\beta^2\) are positive real constants.
Read an article here:
Mathematics Is an Edifice, Not a Toolbox
https://ams.org/notices/199610/page2.pdf