- #1

TimH

- 56

- 0

Anyway, you get the g(a) function which is g(a)=sin(a)/(pi*a) (I'm using "a" for the alpha that appears in the book). I understand this.

Then you want to get your Fourier representation of f(x) so you plug g(a) into the integral from - to + infinity of g(a)*e^(iax) da. This I understand.

Now for this integral she expands e^(iax) using Euler to get cos(ax)+i*sin(ax). If you multiply this by g(a), i.e. by sin(a)/(pi*a), you get two terms in your integral, i.e. the integral is of (sin(a)(cos ax))/(pi*a) + (sin(a)(i*sin(ax))/(pi*a) da.

But she does not show these two terms of the multiplication. Rather, she says that the integral is equal to two times the integral of the first term, because the function "sin(a)/a is an even function." So the complex second term is dropped. This I don't understand. The second term is a function of a and x, times i. I can't really tell if its even or odd. If it's odd, I can see dropping it from the integral since the integral is symmetric around the origin so the integral of the term will be = 0. But the fact that its got an "i" in it is throwing me, too.

Could somebody please help me with this second term in the integral? Thanks.