Let real power series and converge on , with for every index including zero. If has entries there, applying within two diagonal blocks and across them preserves positive semidefiniteness. Indeed the coefficient matrix is a bipartite block-constant positive semidefinite matrix. Each is positive semidefinite by the Schur product theorem, and their sum converges to the transformed matrix. Closedness of the positive semidefinite cone completes the proof. Convergence at one gives , so coefficient domination ensures absolute convergence of both series throughout the interval. Without the zero-index condition, , and give a counterexample.
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