# Why are even/odd functions called even/odd?

Bit of a silly question, someone told me that the reason even functions are called 'even' and odd functions are called 'odd' is that all (single-variable) monomials with even powers are even functions and all monomials with odd powers are odd functions. In addition, this implies that the MacLaurin series (if it exists) of the odd part of a function is a linear combination of monomials with odd powers and similarly for the even case.

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Probably because polynomials and convergent power series in which all terms have even degree are even functions, and similarly for odd.

Later note:

http://jeff560.tripod.com/e.html

quote:

EVEN FUNCTION. Functiones pares is found in 1727 in "Problematis traiectoriarum reciprocarum solutio," presented to the Petersburg Academy in July 1727 by Leonhard Euler:

Primo loco notandae sunt functiones, quas pares appello, quarum haec est proprietas, ut immutatae maneant, etsi loco $x$ ponatur $-x$. [In the first place are noted functions, which I call even, of which there is this property, that they remain unchanged if in place of $x$ is put $-x$.]

end of quote

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@M Turgeon (and Michael Hardy) I agree with you guys, I was just hoping someone could confirm this was the actual origin (i.e., an answer that didn't start with "Probably" or "In my opinion" ^^). – jkn Sep 25 '12 at 15:35
Thanks. It'd be nice to know that it was indeed the $x^n$ story that was going through Euler's mind when he wrote that, but that might be a bit too much to ask for! – jkn Sep 25 '12 at 15:54

I know that the term "even" derives from the fact that the Taylor series of an even function contains only even powers, and so for the odd functions.

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Just beat me to saying that! – Daniel Littlewood Sep 25 '12 at 15:37
That's assuming that the taylor series exists, i.e. that the function is $\mathcal{C}^\infty$, which of course not all even/odd functions are. – jkn Sep 25 '12 at 15:42