Every other neg even number.........like 2pi.....................which starts out.........6.28...........the 1st perfect number......then the 2nd perfect number..........kinda like 1/2.......

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2πr

Animation of the act of unrolling the circumference of a unit circle, a circle with radius of 1. Since C = 2πr, the circumference of a unit circle is 2π.

Unit circle - Wikipedia