even and odd symmetry
A signal is even (symmetric) when x[-n]=x[n], and odd (antisymmetric) when x[-n]=-x[n].
A sequence is even (symmetric) when x[-n]=x[n] -- a mirror image of itself around n=0 -- and odd (antisymmetric) when x[-n]=-x[n], flipped in sign as well as reflected. Every signal decomposes uniquely into an even part plus an odd part.
See also
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