Let $ P $ be a real symplectic matrix satisfying $ P^k = I_{2n} $, where $ k: = \min\{m\in \mathbf{N}^{\ast}\mid P^m = I_{2n}\} $. In this paper, we study the minimal $ P $-symmetric period problem of autonomous and semi-positive definite Hamiltonian systems in $ \mathbf{R}^{2n} $. Applying the iterative inequality and Hörmander index estimate of $ P $-index, we prove that for every $ \tau>0 $, there exists a non-constant $ P $-solution with minimal $ P $-symmetric period not less than $ \frac{\tau} {2n+1} $.
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