Is it permissible to suffice with a single takbeer to join the Imam while he is prostrating?

The opening Takbeer (Takbeerat al-Ihram) is a pillar of prayer, without which the prayer is invalid. It is obligatory upon entering the prayer. It is sunnah for the congregant (ma'mum) to say a second Takbeer for prostration if he joins the Imam while he is prostrating. If he does not say it, his prayer is still valid.