The form envelope of a differential p-form 
in the exterior
algebra 
is the smallest subspace 
such that 
is in the subspace 
. Alternatively, 
is spanned by the vectors that can be written as the tensor
contraction of 
with an element of 
.
For example, the envelope of 
in 
is 
, and the envelope of 
in 
is all of 
.
