The finite series a₁+a₂+…+a_n can be written, using the capital Greek letter sigma, as

Similarly, for example,

(The letter ‘r’ used here is a dummy variable and so could be replaced by any other letter.) In a similar way, the infinite series a₁+a₂+…can be written as

The infinite series may not have a sum to infinity or limit but, if it does, the same abbreviation is also used to denote this limit