The paper generalizes some of our previous results on quasi-hereditary Koszul algebras to graded standardly stratified Koszul algebras. The main result states that the class of standardly stratified algebras for which the left standard modules as well as the right proper standard modules possess a linear projective resolution - the so called linearly stratified algebras - is closed under forming their Yoneda extension algebras. This is proved using the technique of Hilbert and Poincare series of the corresponding modules.
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