Journal of Inequalities and Applications 
Volume 2006 (2006), Article ID 43465, 15 pages
doi:10.1155/JIA/2006/43465

Inequalities involving the mean and the standard deviation of nonnegative real numbers

Oscar Rojo

Departamento de Matemáticas, Universidad Católica del Norte, Antofagasta 1280, Casilla, Chile

Received 22 December 2005; Revised 18 August 2006; Accepted 21 September 2006

Abstract

Let m(y)=j=1nyj/n and s(y)=m(y2)m2(y) be the mean and the standard deviation of the components of the vector y=(y1,y2,,yn1,yn), where yq= (y1q,y2q,,yn1q,ynq) with q a positive integer. Here, we prove that if y0, then m(y2p)+(1/n1)s(y2p)m(y2p+1)+(1/n1)s(y2p+1) for p=0,1,2,. The equality holds if and only if the (n1) largest components of y are equal. It follows that (l2p(y))p=0, l2p(y)=(m(y2p)+(1/n1)s(y2p))2p, is a strictly increasing sequence converging to y1, the largest component of y, except if the (n1) largest components of y are equal. In this case, l2p(y)=y1 for all p.