Stolarsky mean

For two positive real numbers ${\displaystyle x}$ and ${\displaystyle y}$ the Stolarsky Mean is defined as:

${\displaystyle S_{p}(x,y)=\left\{{\begin{array}{l l}x,&{\text{if }}x=y,\\\left({\frac {x^{p}-y^{p}}{p(x-y)}}\right)^{1/(p-1)},&{\text{otherwise}}.\end{array}}\right.}$

It is derived from the mean value theorem, which states that a secant line, cutting the graph of a differentiable function ${\displaystyle f}$ at ${\displaystyle (x,f(x))}$ and ${\displaystyle (y,f(y))}$, has the same slope as a line tangent to the graph at some point ${\displaystyle \xi }$ in the interval ${\displaystyle [x,y]}$.

${\displaystyle \exists \xi \in [x,y]\ f'(\xi )={\frac {f(x)-f(y)}{x-y}}}$

The Stolarsky mean is obtained by

${\displaystyle \xi =\left[f'\right]^{-1}\left({\frac {f(x)-f(y)}{x-y}}\right)}$

when choosing ${\displaystyle f(x)=x^{p}}$.

• ${\displaystyle \lim _{p\to -\infty }S_{p}(x,y)}$ is the minimum.
• ${\displaystyle S_{-1}(x,y)}$ is the geometric mean.
• ${\displaystyle \lim _{p\to 0}S_{p}(x,y)}$ is the logarithmic mean. It can be obtained from the mean value theorem by choosing ${\displaystyle f(x)=\ln x}$.
• ${\displaystyle S_{\frac {1}{2}}(x,y)}$ is the power mean with exponent ${\displaystyle {\frac {1}{2}}}$.
• ${\displaystyle \lim _{p\to 1}S_{p}(x,y)}$ is the identric mean. It can be obtained from the mean value theorem by choosing ${\displaystyle f(x)=x\cdot \ln x}$.
• ${\displaystyle S_{2}(x,y)}$ is the arithmetic mean.
• ${\displaystyle S_{3}(x,y)=QM(x,y,GM(x,y))}$ is a connection to the quadratic mean and the geometric mean.
• ${\displaystyle \lim _{p\to \infty }S_{p}(x,y)}$ is the maximum.