Question: is null space a subspace?
Answer: The null space of an m n matrix A, written as Nul A, is the set of all solutions to the homogeneous equation Ax 0. The null space of an m n matrix A is a subspace of Rn. Equivalently, the set of all solutions to a system Ax 0 of m homogeneous linear equations in n unknowns is a subspace of Rn.