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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
And also don't overlook free resources. While the quality might be significantly more variable, and it's unlikely it'll be the required text for a course you take, in the right places there's plenty of stuff to look into (e.g. here for math).
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.