Every phd student of Terence Tao is probably welcome at the Max Planck Institute For Mathematics:
https://www.mpim-bonn.mpg.de/application
Usually you need 2 letters of recommendation, I’d strongly assume that in the case of Tao one is enough.
Generally please consider the new „ Max Planck Transatlantic Program“:
https://www.mpg.de/25034916/max-planck-transatlantic-program
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