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I have searched google for an answer but I'm not sure what I'm asking. I know that Sigma means sum but there is an 'n' above Sigma and an 'i=1' under sigma. how can i understand this? thank you!

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    $\begingroup$ Shorthand notation for "$a_1+a_2+\cdots+ a_n$" is "$\sum\limits_{i=1}^n a_i$". You would read that summation symbol as "the sum from $i=1$ to $n$ of $a_i$". $\endgroup$ – David Mitra Aug 27 '13 at 15:16
  • $\begingroup$ Ok so it's just a counting sequence! $\endgroup$ – Hermes Trismegistus Aug 27 '13 at 15:18
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    $\begingroup$ @DavidMitra I know what you mean, but just so this doesn't go without saying, it's $a_1+a_2+\ldots +a_n$ that is short for $\sum \limits_{i=1}^na_n$. $\endgroup$ – Git Gud Aug 27 '13 at 15:20
  • $\begingroup$ @GitGud You do realize $a_1+\cdots+a_n$ is really "informal" notation for an otherwise well defined and compact $\sum_{i=1}^n a_i$? $\endgroup$ – Pedro Tamaroff Aug 28 '13 at 3:10
  • $\begingroup$ @PeterTamaroff That's exactly what my comment above conveys. $\endgroup$ – Git Gud Aug 28 '13 at 9:04
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The n is the variable it is being summed up to. The i=1 is the starting term. The expression to the right of the sigma notation is the expression being summed from 1 to n.

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  • $\begingroup$ Great! That's easy. The book didn't explain it at all and it's the first chapter. $\endgroup$ – Hermes Trismegistus Aug 27 '13 at 15:18
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$\sum_{i=m}^n a_i = a_m + a_{m+1} + a_{m+2} +\cdots+ a_{n-1} + a_n.$

Where, i represents the index of summation; a_i is an indexed variable representing each successive term in the series; m is the lower bound of summation, and n is the upper bound of summation. The "i = m" under the summation symbol means that the index i starts out equal to m. The index, i, is incremented by 1 for each successive term, stopping when i = n.

Read this for better understanding.

http://en.wikipedia.org/wiki/Summation

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