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Sigma
The Greek capital letter sigma ∑ is the symbol for a sum of similar terms.
Explanation
A summation with the variable x is written as
The number i is called the index. The number m is the lower bound of the summation, the number n is its upper bound.
Example 1
The sum of the binomial theorem reads
and written out it gives
Example 2
The Basel problem asks for the exact value of the infinite series
The calculation yields a formula for π, because the solution is
