Detecting if a polynomial is a Pfaffian Given an explicit polynomial, is there any kind of trick/algorithm to check whether it is a pfaffian of a matrix with linear entries? 
The pfaffian can be defined as $\sqrt{{\rm det}(A) } $ when $A$ is skew symmetric, or explicitly $${\rm pf}(A) = \frac{1}{2^n n!}\sum\limits_{\sigma \in S_{2n}}{\rm sgn} (\sigma)\prod\limits_{j=1}^n a_{\sigma (2j -1 ),\sigma(2j)}.$$ 
 A: Now that the question makes sense, let me mention the following result (you can find it in Eisenbud, Exercise 20.17) which hinted at what Francesco wrote.
Let $R = k[[x,y,z]]$ and $f\in m=(x,y,z)$. Then $f$ is a determinant of a matrix (of size at least $2$) with entries in $m$ iff $R/(f)$ is not a UFD!    
A: As Bruce Westubury noticed, the answer to this question is trivial as it is stated.
Surprisingly enough, however, the situation becomes very interesting when one considers representations of homogeneous polynomials as pfaffians of matrices with linear entries.
More precisely, let us consider the following version of the question:

Question. Let $F \in k[x_0, \ldots, x_n   ]$ be a homogeneous polynomial of degree $d$.
  Does there exist a symmetric (resp. antisymmetric) matrix $M$, whose entries are linear forms, such that 
  $$\det(M)=F \quad (\textrm{resp}. \  \textrm{Pf}(M)=F)?$$  

This problem is studied in detail in Beauville's paper [Symmetric determinantal hypersurfaces, Michigan Mathematical Journal 48 (2000)], where the existence of a symmetric or pfaffian representation is related to the existence of certain vector bundles on the projective variety $X \subset \mathbb{P}^n_k$ defined by $F=0$.
Among other things, Beauville proves the following results (when $k= \mathbb{C}$):
$\bullet$ A general polynomial of degree $d$ admits a determinantal representation if and only if $$n=2 \quad \textrm{or}$$ $$n=3 \quad \textrm{and} \quad d\leq 3.$$
$\bullet$ A general polynomial of degree $d$ admits a pfaffian  representation if and only if $$n=2,$$ $$n=3 \quad \textrm{and} \quad d \leq 15 \quad \textrm{or}$$   $$n=4 \quad \textrm{and} \quad d \leq 5.$$
A: The Pfaffian of $\begin{pmatrix} 0 & A \\\\ -A & 0\end{pmatrix}$ is $\det A$. Since any polynomial is a determinant this means any polynomial is a Pfaffian.
