In this paper, we show that the minimizing problem Lambda(s,alpha) = inf(u(H) over dots(RN), u not equivalent to 0) integral(RN)vertical bar-Delta(s/2)u(x)vertical bar(2) dx/(integral(RN)vertical bar u(x)vertical bar(2)*(s,alpha)/vertical bar x vertical bar(alpha) dx)(2/2)*(s,alpha) (1) is achieved by a positive, radially symmetric and strictly decreasing function provided 0 < s < N/2, 0 < alpha < 2s. (C) 2014 Elsevier Ltd. All rights reserved.