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Homework Statement
Prove that the intersection of any collection of T-invariant subspaces of V is a T-invariant subspace of V.
Homework Equations
The Attempt at a Solution
Let W1 and W2 be T-invariant subspaces of V. Let W be their intersection.
If v[itex]\in[/itex]W, then v[itex]\in[/itex]W1 and v[itex]\in[/itex]W2. Since v[itex]\in[/itex]W1, T(v)[itex]\in[/itex]W1 & v[itex]\in[/itex]W2, T(v)[itex]\in[/itex]W2. Therefore T(v)[itex]\in[/itex]W.
For any x,y[itex]\in[/itex]W, x,y[itex]\in[/itex]W1 and x,y[itex]\in[/itex]W2, x,y[itex]\in[/itex]W and cx+y[itex]\in[/itex]W since W1 and W2 are subspaces.
Thus the intersection of any collection of T-invariant subspaces is a T-invariant subspace.
My answer was marked wrong. The grader's comment was to cross out "any" and replace it with "2". What I should have said? Was I supposed to explicitly point out that this applies from 2...n? :(