Here is a counterintuitive consequence: the vector space R[t] of polynomials has a
countably infinite linear covering—indeed, for each n ∈ Z
+, let Wn be the subspace of
polynomials of degree at most n. However any irredundant linear covering of R[t] has
cardinality #R + 1 = 2
ℵ0
. Redundant coverings can be much more efficient!
That a finite-dimensional vector space over an infinite field cannot be a finite union
of proper linear subspaces is mathematical folkore: the problem and its solution appear
many times in the literature. For instance problem 10707 in this MONTHLY is intermediate
between this fact and our main result. The editorial comments given on page
951 of the December 2000 issue of the MONTHLY give references to variants dating
back to 1959. Like many pieces of folklore, there seems to a be mild stigma against
putting it in standard texts; an exception is [2, Thm. 1.2]