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Unity.Mathematics
The C# math library used in Unity providing vector types and math functions with a shader like syntax
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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.
So, Unity's Mathematics library is open source: https://github.com/Unity-Technologies/Unity.Mathematics but I don't think the license actually lets you use it in projects that are not in Unity.
NOTE:
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Hence, a higher number means a more popular project.