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Here are a couple of resources explaining what DOI is, when and why created.
Flowchart showing steps to using a DOI and where to locate it.
A DOI URL looks like this ----- https://doi.org/10.1007/s10754-013-9127-4
A general DOI number is -- 10.1007/s10754-013-9127-4
Two links that help with verifying DOIs: