  Let $S_1=\{p_1,p_2,\cdots, p_l\}\subset\cc[z_1,z_2\cdots,z_n]$ be a set of
quadratic forms such that $p_i=q_i^2$ where $\{q_i\}_{i=1}^l$ are linear forms.
For $1\leq k\leq l$, let $S_k=\{p_{i_1}p_{i_2}\cdots p_{i_k}|1\leq
i_1<i_2<\cdots<i_k\leq l\}$ be the set of $k$-products of distinct polynomials
in $S_1$. We show somehow unexpectedly that linear independence of $S_1$ is
equivalent to that of $S_k$ for $k=2$ and $3$ under certain rank conditions.
Among technical treatments in the proof, of independent conceptual interest is
a novel polynomial identity which elegantly incorporates quadratic forms and
matrix determinants.
