What are you trying to do?
You can divide a publickey by 2 but you don't know if the corresponding privatekey is even or odd so it either is 1 bit below or somewhere else...
why only div to 2 ? I can div more.
seevmy ex try div 2^120/2^60 you get 2
2^120/2^60 = 2^60
(2^120-56667788)/2^60 = 2^60
then div real 110 bit pubkey aways get float and in part, but posible subsract int part from resul of dividing and privjey will be after dmaler then div to 2...