From the text: "The Nash inequality states that (1) $$\Big(\int_{\bold R^n}|f(x)|^2\,d^nx\Big)^{1+2/n}\leq C_n\int_{\bold R^n}|\nabla f(x)|^2\,d^nx\Big(\int_{\bold R^n}|f(x)|\,d^nx\Big)^{4/n}$$
(∫Rn|f(x)|2dnx)1+2/n≤Cn∫Rn|∇f(x)|2dnx(∫Rn|f(x)|dnx)4/n
for a constant $C_n$Cn depending only on $n$n. This inequality is a particular case of the Gagliardo-Nirenberg inequalities for which numerous applications have been found. In this note we compute the sharp constant in (1) and determine all of the cases of equality. A particularly striking feature of the result is that all of the extremals have compact support.''
With the assumption of the minimizer is obtained for radially decreasing function, choose a ball, whose radius is given by self-consistence conditions.