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Let $K$ be a field, $1 \leq d \leq n$ integers and $V$ an $n$-dimensional vector space. The Plücker relations are quadratic forms on $\wedge^d V$ whose zero set is exactly the set of decomposable vectors in $\wedge^d V$ (i.e. which are of the form $v_1 \wedge ... \wedge v_d$), thus describing the ideal corresponding to the Plücker embedding $\text{Gr}_d(V) \to \mathbb{P}(\wedge^d V)$. But in every book I've read so far, these Plücker relations are constructed by means of many identifications between duals, exterior powers, etc. so that I am not able to write them down explicitely. Although I've tried it, many signs and sums confuse me.

Question. Is it possible to write down these Plücker relations explicitely as a set of polynomials in the ring $K[\{x_H\}]$, where $H$ runs through the subsets of $\{1,...,n\}$ with $d$ elements? (Of course it is possible, but I wonder how do this in general)

Edit: Following the answer below, here is the

Answer: Instead of using these subsets $H$, use indices $1 \leq i_1 < ... < i_d \leq n$, and extend the definition of $x_{i_1,...,i_d}$ to all $d$-tuples in such a way that $x_{i_1,...,i_d}=0$ if these $i_j$ are not pairwise distinct, and otherwise $x_{i_1,....,i_d} = sign(\sigma) \cdot x_{i_{\sigma(1)},...,i_{\sigma(d)}}$, where $\sigma$ is the unique permutation of $1,...,d$ which makes $i_{\sigma(1)} < ... < i_{\sigma(d)}$. Then the Plücker relations are

$\sum\limits_{j=0}^{d} (-1)^j x_{i_1,...,i_{d-1},k_j} * x_{k_0,...,\hat{k_j},...,k_d} = 0$

for integers $i_1,...,i_{d-1},k_0,...,k_d$ between $1,...,n$.

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    You suggest that the projective Grassmannian $\mathbb G(3,7)$ of $3$-planes in $\mathbb P^7$ is a quadric in $\mathbb P(\wedge^8 K^4)$ (because the Plücker relations generate the ideal of relations of the Grassmannian and you suggest there's only one relation, a quadric), thus of projective dimension $(\binom 84 - 1)-1 = 68$. We know that the dimension of $\mathbb G(p,n)$ is $(p+1)(n-p)$, so no, in this case you miscounted : The dimension of $\mathbb G(3,7)$ is $(3+1)(7-3) = 16$. So you missed a lot of quadrics!2014-11-02

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Yes, the Plücker relations are written down totally explicitly in terms of the polynomials you require on page 110, equation (3.4.10), of Jacobson's book Finite-Dimensional Algebras over Fields. The proof, attributed by the author to Faulkner (a student of his?), is completely down-to-earth: no identifications, no duality,...

Edit Since Martin doesn't have access to the book, I'm adding an online presentation, with the relevant equations on page 21. It is very elementary, with concrete examples, and might appeal to readers whose interest has been whetted by Martin's question.
And the bibliography contains a reference to a masterful article by Kleiman and Laksov, which also contains the Plücker relations handled with minors of determinants and nothing else.

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    The reference to the article by Kleiman-Laksov is great, thank you.2011-07-03
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I think the answer provided above could be improved a bit. (Not logically, just made a bit clearer.) My method avoids having extra variables and setting repeating sequences equal to zero.

Take a field $F$, and let $X = \{x_H : H \subset \{1,2,...n\} \text{ and } |H| = d\}$ So we can think of variables as being indexed by length $d$ increasing sequences. For $I,K \subset \{1,2,...,n\}$ of size $d-1$ and $d+1$, respectively, we can define the $(I,K)$ Plucker relation $Pl_{I,K}(X)$ by the formula:$ Pl_{I,K}(X) = \sum_{k \in K - I} (-1)^{S_{I,K}(k)} x_{I \cup \{k\}} x_{K - \{k\}} $ And the sign $S_{I,K}(k) = \#\{i \in I : k < i\} + \#\{\ell \in K: k < \ell\} $

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Let $B_r\in G_n^r$ be an $r$-vector in the Clifford geometric algebra $G_n$ of the Euclidean vector space $R^n$. In this setting, the Pl\"ucker relations take the elegant coordinate-free form $(A_{r-1}\cdot B_r)\wedge B_r=0$ for all $(r-1)$-vectors $A_{r-1}\in G_n^{r-1}$. When the quadratic homogeneous Pl\"ucker relations are satisfied, the $r$-vector $B_r$ can be factored into the outer product of $r$ vectors $v_1, \cdots, v_r\in G_n^1$, and an explicit formula can be given for these vectors. I have just today published the paper {\it Notes on Pl\"ucker's Relations in Geometric Algebra}. It can be found on my website: http://www.garretstar.com