bitcoin halving != bitcoin value increase
in the sense that the value will not come on the same date, it could come at the end of this year for all we know
I wouldn't be very surprised if we'll actually see a drop in the price on/close to halving day. But, if there are no other significant events, halving should definitely drive the price up in the longer term.
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I presume the difficulty will decrease because many people will not be able to pay for the electricity mining only half of what they are mining now.
Until the price doubles, then they will turn on their rigs again.
Well, remember that we had BTC in $200 range for quite a long time and it didn't cause any massive reduction in hashing power (slight drop iirc). So anything above $450 should be sufficient.