r/MathHelp • u/rumbi02 • Mar 17 '21

Sigma Notation

Hey guys, quick question, when doing sigma notation for series/sequences, can the n ever be a negative? Or must it always be positive? Thank you :)

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3

u/Shadowsca Mar 17 '21

Yeah it can be anything as long as the set of possible values is finite or countably infinite.

As as example you can have Sigma_{n in N} 1/2n

or Sigma_{-10 <= n <= 10} 1/2n

2

u/hausdorffparty Mar 17 '21

And well ordered. You have to be really careful summing over the rational numbers for example :)

2

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2

u/Jantesviker Mar 17 '21

Sure. Try to write (-4) + (-3) + (-2) + (-1) in sigma notation.

1

u/isabelisnthere Mar 17 '21

Yeah. n is just the first value substituted into the summation