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From the Cauchy-Schwarz inequality, we can prove that $\lVert w(x)\rVert^2_{L^2_{[0,1]}}=\int_0^1 w(x)^2\, dx \leq \sqrt{\int_0^1 w(x) \,dx}\cdot \sqrt{\int_0^1 w(x)^3\, dx}.$

Is it possible to prove another inequality with the other direction, that is, $\int_0^1 w(x)^3\, dx \leq C_1\cdot \left(\int_0^1 w(x)^2 dx\right)^{1/n}\;?$

Thanks a lot!!!

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    :I already gave you the constant $C$ under certain conditions in my answer.2012-08-16

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No, because $\int_0^1 w(x)^3\ dx$ can be infinite while $\int_0^1 w(x)^2 \ dx$ is finite. Consider e.g. $w(x) = x^{-1/3}$.

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    If you want $C_1$ to be a constant, and you allow, for example, all polynomials, then it is impossible, because you can e.g. take a polynomial that uniformly approximates $\min(R, x^{-1/3})$ for arbitrarily large $R$.2012-08-16