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Summation notation

Sigma, Σ, is the standard notation for writing long sums. Learn how it is used in this video. Created by Sal Khan.

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  • spunky sam blue style avatar for user HotRodFire2000
    At , shouldn't the i to the right of the sigma be i+1?
    (23 votes)
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    • blobby green style avatar for user Creeksider
      No, it has to be i. We're adding the numbers from 1 to 10. The sigma notation says we're going to add up the results of applying the rule to the right of the symbol to each of the i's from 1 to ten. In this case we're applying a rule that does nothing, just gives back i. So the first result in the addition is 1, then 2 and so on up to 10. If we had i+1 to the right of the symbol, the first result in the addition would be 2, and we would end up adding the numbers from 2 to 11.
      (116 votes)
  • winston default style avatar for user David
    At the end of the video, I'm just wondering could the index be a decimal? If so, what if the number on top of the sigma was a integer? How would that work? I'm just curious and brand new to sigma. Tell me if my question is completely unreasonable.
    (44 votes)
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  • starky tree style avatar for user Darcy Lynch
    So, is sigma notation is kind of like a for loop in programming?
    (51 votes)
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    • blobby green style avatar for user Hexagon
      As a while loop in python:
      import math
      i = 0 #starting index
      n = 0 #result of that index number
      s = 0 #total sum
      while i <= 50: #ending index
      n = math.pi*i**2
      print(str(i) + "|" + str(n))
      i += 1
      s = s + n
      print("sum: " + str(s))
      (4 votes)
  • blobby green style avatar for user Nikhil Bhatia
    what happens if it is constant summation
    (24 votes)
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  • blobby green style avatar for user Tobey
    So by default, as in when you don't put anything else in front of the i next to the Greek letter, whatever i is set as equal to below the letter increases by 1 in the summation? So that when you want to set the summation as going 1+2+3+4+5+6+7+8+9+10 you just write down i next to the Greek letter, then when you put, say, 2 next to the i as in 2i, the summation goes 2+4+6+8+10+12+14+16+18+20 and not 2+2+2+2+2+2+2+2+2+2?
    (12 votes)
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  • starky sapling style avatar for user Helix8
    Can we set the index as a negative number? Also, can the upper boundary be a negative number?
    (8 votes)
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    • blobby green style avatar for user niebau
      The definition of the sigma notation seems to be surprisingly vague, and I couldn't find anything that would explicitly prohibit the usage of negative indices or negative boundaries, as long as they are used in a consistent way (lower boundary < upper boundary).
      However, I would rather avoid doing so for several reasons:

      1) The sigma notation basically represents the terms of a series, and each term is usually associated with a letter and the corresponding index (e.g. a1 for the first term, a2 for the second one, and so on). It would be more than unconventional to use negative indices for these terms (e.g. a-5).

      2) I've never seen the usage of negative indices or boundaries in any text book. Since the sigma notation is basically just a convention to write out long sums in a short way, it's probably best to stick to the prevailing convention of using non-negative indices and boundaries only.

      Summarized, I wouldn't go as far as calling the usage of negative indices and boundaries as 'wrong', but it will at least raise some eye brows.
      (17 votes)
  • blobby green style avatar for user Ryujin Jakka
    Is there a video out there with a more in depth explanation on how to use summation notation, especially with integrals? Unfortunately, the first time I ever saw summation notation was in the heat of integral calculus a few weeks ago. I am finding it extremely difficult to understand all of the different symbols and how they are being used...
    (7 votes)
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  • aqualine ultimate style avatar for user Roy
    If I wanted the increment to be more than 1, would there be a specific notation for that, or would I just have to implement it into the right - hand side of the Sigma notation?
    (5 votes)
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  • piceratops ultimate style avatar for user Dave Morgan
    How do you express sigma notation when typing?
    (5 votes)
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  • piceratops ultimate style avatar for user ET
    What about pi notation, why is sigma notation so much moer popular, just curious (pi notation is a capital pi and is used for sequences of constant mutiplying, instead of adding like sigma is)? Thx
    (3 votes)
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Video transcript

What I want to do in this video is introduce you to the idea of Sigma notation, which will be used extensively through your mathematical career. So let's just say you wanted to find a sum of some terms, and these terms have a pattern. So let's say you want to find the sum of the first 10 numbers. So you could say 1 plus 2 plus 3 plus, and you go all the way to plus 9 plus 10. And I clearly could have even written this whole thing out, but you can imagine it becomes a lot harder if you wanted to find the sum of the first 100 numbers. So that would be 1 plus 2 plus 3 plus, and you would go all the way to 99 plus 100. So mathematicians said, well, let's find some notation, instead of having to do this dot dot dot thing-- which you will see sometimes done-- so that we can more cleanly express these types of sums. And that's where Sigma notation comes from. So this sum up here, right over here, this first one, it could be represented as Sigma. Use a capital Sigma, this Greek letter right over here. And what you do is you define an index. And you could start your index at some value. So let's say your index starts at 1. I'll just use i for index. So let's say that i starts at 1, and I'm going to go to 10. So i starts at 1, and it goes to 10. And I'm going to sum up the i's. So how does this translate into this right over here? Well, what you do is you start wherever the index is. If the index is at 1, set i equal to 1. Write the 1 down, and then you increment the index. And so i will then be equal to 2. i is 2. Put the 2 down. And you're summing each of these terms as you go. And you go all the way until i is equal to 10. So given what I just told you, I encourage you to pause this video and write the Sigma notation for this sum right over here. Assuming you've given a go at it, well, this would be the sum. The first term, well, it might be easy to just say we'll start at i equals 1 again. But now we're not going to stop until i equals 100, and we're going to sum up all of the i's. Let's do another example. Let's imagine the sum from i equals 0 to 50 of-- I don't know, let me say-- pi i squared. What would this sum look like? And once again, I encourage you to pause the video and write it out, expand out this sum. Well, let's just go step by step. When i equals 0, this will be pi times 0 squared. And that's clearly 0, but I'll write it out. pi times 0 squared. Then we increase our i. And, well, we make sure that we haven't hit this, that our i isn't already this top boundary right over here or this top value. So now we said i equals 1, pi times 1 squared-- so plus pi times 1 squared. Well, is 1 our top value right over here, where we stop? No. So we keep going. So then we go i equals 2, pi times 2 squared-- so plus pi times 2 squared. I think you see the pattern here. And we're just going to keep going all the way until, at some point-- we're going to keeping incrementing our i. i is going to be 49. So it's going to be pi times 49 squared. And then finally we increment i. i equal becomes 50, and so we're going to have plus pi times 50 squared. And then we say, OK, our i is finally equal to this top boundary, and now we can stop. And so you can see this notation, this Sigma notation for this sum was a much cleaner way, a much purer way, of representing this than having to write out the entire sum. But you'll see people switch back and forth between the two.