Levinson's inequality

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In mathematics, Levinson's inequality is the following inequality, due to Norman Levinson, involving positive numbers. Let a>0 and let f be a given function having a third derivative on the range (0,2a), and such that

f'''(x)\geq 0

for all x\in (0,2a). Suppose 0<x_i\leq a for  i = 1, \ldots, n and 0<p. Then

\frac{\sum_{i=1}^np_i f(x_i)}{\sum_{i=1}^np_i}-f\left(\frac{\sum_{i=1}^np_ix_i}{\sum_{i=1}^np_i}\right)\le\frac{\sum_{i=1}^np_if(2a-x_i)}{\sum_{i=1}^np_i}-f\left(\frac{\sum_{i=1}^np_i(2a-x_i)}{\sum_{i=1}^np_i}\right).

The Ky Fan inequality is the special case of Levinson's inequality where

p_i=1,\  a=\frac{1}{2},


f(x)=\log x. \,


  • Scott Lawrence and Daniel Segalman: A generalization of two inequalities involving means, Proceedings of the American Mathematical Society. Vol 35 No. 1, September 1972.
  • Norman Levinson: Generalization of an inequality of Ky Fan, Journal of Mathematical Analysis and Applications. Vol 8 (1964), 133–134.