Let $q$ denote a quadratic form over a field $k$.


The u-invariant of a field $u(k)$ is defined by $u(k):=\{ max (\mathrm{rank}(q)) $ | $ q $ is anisotropic over $k\}$.


Let $k = \mathbb{Q}\_p$ for any prime $p$ and set


$L = k(t\_1,..,t\_n)$.


It is known that $u(k)=4$ and newer results by David B. Leep state that 


$u(L) = 4\cdot2^n = 2^{n+2}$.


As a consequence from the Arason-Pfister Hauptsatz we have that


$2^{cd(L)} \leq u(L) = 2^{n+2}$, while $cd(L)$ denotes the cohomological dimension of $L$.


Is $cd(L) = n+2$ i.e. does equality hold in the upper equation?


This question is not trivial in general as Serre points out by mentioning results of Merkurjev in Galois Cohomology. Merkurjev constructs fields $k$ with $cd(k)=2$ having any desired even $u(k) \geq 2$.



Cohomological dimension of transcendental p-adic extensions
I think the answer to your question can be found in Serre's "Galois cohomology" book, Section II.4.2, where a more general result is proved for transcendental field extensions.


