In the study of a particular PDE I found myself wanting to prove the following inequality:


$( \int\_0^{\infty} r^{-3} |f|^6 \; dr )^{1/6} \leq C ( \int\_0^{\infty} [ r^{-1} |f|^2 + r |f'|^2 + r |f''|^2] \; dr )^{1/2}$


for some constant $C > 0$ and all $f \in C^{\infty}\_c((0,\infty);(-\infty,\infty))$.


Partial progress:


1. If $\mathop{supp} f \in [r\_0, \infty)$, for some $r\_0 > 0$ then
$( \int\_0^{\infty} r^{-3} |f|^6 \; dr )^{1/6} \leq C(r\_0) ( \int\_0^{\infty} [ r^{-1} |f|^2 + r |f'|^2 ] \; dr )^{1/2}$.
However, $C(r\_0) \rightarrow \infty$ as $r\_0 \searrow 0$ as can be seen by basic scaling. This lead me to try to decompose $f$ into two parts, for concreteness, one supported on say $[1/2,\infty)$ and the other supported in $(0,2]$. However, this didn't lead me anywhere.
2. I employed a heuristic analysis as in <http://terrytao.wordpress.com/2010/03/11/a-type-diagram-for-function-spaces/> . Of course, one has to also track the position of the bump function since the norms are not translation invariant. The inequality was satisfied under this heuristic but I didn't know how to make it rigourous.
3. Tried the change of variables $p = \log r$ to no avail.


I don't have any reason, except for the above, to believe that the inequality is true so perhaps someone can come up with a counterexample?


Any help would be much appreciated, thanks in advance.



A Sobolev-type inequality with weights
To complete your computation, let's treat the case of a function supported in interval $(0,1)$. Indeed, for $ f\in C^\infty\_c(0,1)$ there is an inequality 
$$ \int\_0^1 r^{-3}f(r)^6 dr\le C\left(\int\_0^1 rf''(r)^2dr \right)^3\, .$$


For any $ f\in C^\infty \_ c(\mathbb{R} \_ + )$, the Hardy inequality with exponent $3$, applied to the function $f(r)^2$, gives
$$\int\_0^\infty r^{-3}f(r)^6 dr=\int\_0^\infty\left( \frac{f(r)^2}{r}\right)^3 dr\le 3^3\int\_0^{\infty }f(r)^3f'(r)^3 dr\, .$$
By the Cauchy-Schwarz inequality
$$\left( \int\_0^{\infty }f(r)^3f'(r)^3 dr \right)^2\le \left( \int\_0^{\infty }r^{-3}f(r)^6 dr \right) \left( \int\_0^{\infty }r^3f'(r)^6 dr \right) $$
so that 
$$\int\_0^\infty r^{-3}f(r)^6 dr\le 3^6\int\_0^{\infty }r^3f'(r)^6 dr \, .\qquad\qquad(1)$$
Now assume $\operatorname{supp}(f)\subset(0,1)$, and let $u\in C^{\infty}(\mathbb{R} \_ +)$ be the composition $u(p):=f'(e^{-p})$. With the change of variable $p:=-\log r$, the latter integral writes
$$\int\_0^1 r^3f'(r)^6 dr= \int\_0^{\infty } e^{-4p}u(p)^6 dp\, .\qquad\qquad (2)$$
Since $u(0)=0$, we have the well-known pointwise bounds on $u$
$$|u(p)|\le \int\_0^p|u'(p)| dp\le p^{1/2}\left(\int\_0^{\infty}|u'(p)|^2 dp \right)^{1/2}$$
so, by **(2)**, returning to the $r$ variables
$$\int\_0^1 r^3f'(r)^6 dr\le\left(\int\_0^{\infty} p^3 e^{-4p} dp \right) \left(\int\_0^{\infty}|u'(p)|^2 dp \right)^3 =\frac{3}{2^7}\left(\int\_0^1 rf''(r)^2dr \right)^3\, .$$
The latter, together with inequality **(1)** proves the claim with $C:=(3/2)^7\, .$


