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I'm in the process or preparing a lecture on cryptocurrency and have studied dozens of sources. I have a pretty good understanding of most aspects of the bitcoin blockchain scheme. However, all of them have skipped over several points that seem important to me. Here's two:

  1. When does a miner submit his block for verification? Is it when the when the number of transactions reached a certain number, when their total file size reached a certain value or some other criterion?

  2. Does the miner have any flexibility in choosing the transactions to include in his block? Why not just big ones to reach the limit quicker?

  3. Can there be more than one block circulating in the network seeking verification at the same time?

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  1. As soon as the miner finds a solution to the proof of work. This is as soon as they find a combination of transactions and other data for which the hash is less than, or equal to, the current target.
  2. Yes, the miner can choose whatever transactions maximise their return. Blocks can contain zero transactions (other than the coinbase transaction which pays the miner their mining reward).
  3. Yes, there can be forks in the blockchain. Mostly resolved very quickly. Chain with most work wins, others discarded.
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  • Most difficult chain wins, not longest.
    – Claris
    Mar 7 '20 at 0:51
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    @Anonymous: Thanks, edited. Mar 7 '20 at 0:54
  • Can a block contain zero transactions, or is it necessary to have a coinbase transaction? I'm aware of blocks with only a coinbase, but none without.
    – Mark H
    Mar 7 '20 at 10:56
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    There will still be a coinbase tx, which collects the transaction fees for all transactions in the block. Only difference is that there will be no subsidy added to the amount.
    – Mark H
    Mar 7 '20 at 18:08
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    Blocks without coinbase are unconditionally invalid, even after the subsidy runs out. Transaction outputs can have value 0, so this is not a problem even absent fees (such outputs are nonstandard due to the dust rule, but miners are not subject to those). Mar 7 '20 at 19:04

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