Suppose we have a ring $ R $ and some $ R-$module $ N. $ Let $ a \in \bigwedge^{k} N, $ and $ b \in \bigwedge^{l} N. $
Why is it that $ a \wedge b = (-1)^{kl} \; b \wedge a? $
Suppose we have a ring $ R $ and some $ R-$module $ N. $ Let $ a \in \bigwedge^{k} N, $ and $ b \in \bigwedge^{l} N. $
Why is it that $ a \wedge b = (-1)^{kl} \; b \wedge a? $
If $a = e_1 \wedge e_2 \wedge \ldots \wedge e_k$, and $b = f_1 \wedge f_2 \wedge \ldots \wedge f_l$, then in order to rearrange $a \wedge b$ into $b \wedge a$, I have to move each of the $l$ terms $f_1, \ldots, f_l$ "across" the $k$ terms $e_1, \ldots, e_k$. Each time I move an $f$ "across" an $e$, I introduce a minus sign. So in all there are $kl$ minus signs introduced.
By moving "across", I mean using the identity $e_i \wedge f_j$ = $-f_j \wedge e_i$.