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The symbol looks like a mirrored capitalized E.
Because I don't know its name, I cannot Google for it.

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    It's a fair question since you would have to know Greek or math notation. Can be found here here, and here. – Peter Feb 16 '16 at 13:46
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    Note that almost all math symbols are Greek letters, so if you find another one you want to know the name of, just check the Greek alphabet: en.wikipedia.org/wiki/Greek_alphabet#Letters – stackexchanger Feb 16 '16 at 17:17
  • Lists of mathematical symbols also help. E.g., the capital sigma for summation that you're looking for is available in this list. Something like Detexify may help, too, although it won't necessarily tell you the name of the symbol, it may well help. – Joshua Taylor Feb 16 '16 at 18:59
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    Your description "mirrored capitalized E" would more accurately describe the existential quantification operator than the summation operator IMHO – steeldriver Feb 16 '16 at 19:10
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    Not relevant to the question, which is about the English language, but the equation in the question is incorrect (unless N=0). For example, taking k=2 makes the claim "N/2+N/4 = 2N", which is clearly false. – David Richerby Feb 17 '16 at 0:03

It is an upper case Sigma, from the Greek alphabet.

It is almost always used to denote a summation:

and is therefore (technically) called a summation symbol, as others pointed out in the comments.

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    Yes! Therefore the usual English name of this mathematical symbol (in this context) is summation symbol, summation sign, or summation operator. The related Unicode character for this use is ∑ (U+2211) and called n-ary summation (where "n-ary" is a generic term generalizing "unary", "binary", "ternary" etc.). In contrast the usual Greek letter has another character Σ (U+03A3) which is called Greek captal letter sigma. – Jeppe Stig Nielsen Feb 16 '16 at 15:57
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    And read out loud (as part of a complete phrase, not pointing out the symbol itself) it's simply "sum". As in "The sum from k equals 0 to 5 of k" – hobbs Feb 16 '16 at 23:40
  • While it's true that the symbol is a Greek capital sigma, it is much more appropriate to describe it as a summation sign. – David Richerby Feb 16 '16 at 23:59
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    @hobbs I thought it was "the sum of k from k equals 0 to 5"? – tox123 Feb 17 '16 at 3:07
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    @lovespring yes, it should be either this, or "I don't know it's name, so I cannot Google for it". The words "because" and "so" have the same function in this context. – Glorfindel Feb 17 '16 at 7:43

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