Why was 21 million picked as the number of bitcoins to be created?
I think im right but it is the number that closely satisfied block reward sums
But then why did satoshi set the initial block reward at 50 btc and block halving at 210000 blocks (~4 years)?
There is a thread about the bitcoin magic numbers (https://bitcointalk.org/index.php?topic=263750.0), but there are not much discussion there.
