Let $f(x)$ be a real, periodic function satisfying $f(-x)=-f(x)$. The general form of its Fourier series representation would be
1. $f(x)=a_{0}+\sum_{k=1}^{\infty}a_{k}\cos(kx)$
2. $f(x)=\sum_{k=1}^{\infty}b_{k}\sin(kx)$
3. $f(x)=a_{0}+\sum_{k=1}^{\infty}a_{2k}\cos(kx)$
4. $f(x)=\sum_{k=0}^{\infty}a_{2k+1}\sin(2k+1)x$