Unfortunately, there is no explanation in the math book. This is the18th Greek letter, pronounced "Sima". It is used to represent "area density" in physics and "summation symbol" in mathematics.

Usage is as follows:

10

σI = 1+2+3+4+5+6+7+8+9+ 10 = 55

i= 1

Where "10" represents the upper bound of the sum, and "i= 1" represents the lower bound of the sum, that is, integer accumulation greater than or equal to the lower bound and less than or equal to the upper bound is taken during the operation.

References:

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