Question

What does it mean when there is nothing on top of sigma?

Answer 1

The upper limit of summation is usually omitted when the index is obvious from the context.

Example: If we know
`a_n` is an infinite sequence (i.e. a function
`a_n:\mathbb N\to\mathbb R`, for instance), then we can denote the summation of all the terms in the sequence by


`\displaystyle\sum_(n=1)^\infty a_n`

or by


`\displaystyle\sum_n a_n`