After the birth of a pulsar, its spin frequency will slow down by the EM emission if the magnetic dipole rotating model is assumed. With the present spin period and its derivative of the pulsar, we cannot know exactly the initial period, unless the age of pulsar is given. The spin evolution history of Crab pulsar and some neutron stars, magnetars (SGR/AXP) for instance, with the estimated SNR ages can be evaluated, by which we investigated their initial period and future period.
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