From Summation.

Without {{math}}

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Mathematical notation uses a symbol that compactly represents summation of many similar terms: the summation symbol, Σ, an enlarged form of the upright capital Greek letter sigma. This is defined as

nΣi = m ai = am + am + 1 + am + 2 + ... + an - 1 + an

where i is the index of summation; ai is an indexed variable representing each term of the sum; m is the lower bound of summation, and n is the upper bound of summation.

With {{math}}

edit

Mathematical notation uses a symbol that compactly represents summation of many similar terms: the summation symbol, Σ, an enlarged form of the upright capital Greek letter sigma. This is defined as

nΣi = m ai = am + am + 1 + am + 2 + ... + an - 1 + an

where i is the index of summation; ai is an indexed variable representing each term of the sum; m is the lower bound of summation, and n is the upper bound of summation.