root construction error so you take a vector Y but you know project it on the principle subspace which is done by this matrix W and then multiply by W transpose and assuming things are appropriately normalized you get a reconstruction of the original point which is just in the original space the location of a projection of a point onto the onto the mat the linear subspace the principal subspace so this is the principal subspace here in dark and you take any point you project it here and the energy now the reconstruction error