According to Griffiths QM book, after he derived the stationary state solutions to the Schrodinger equation for a particle in an infinite potential well, which are just functions of sine, he claims that these stationary solutions are orthogonal and complete. I agree that they are orthogonal (since sin(nx) and sin(mx) are orthogonal for n!=m), but I definately disagree that they are complete (meaning that ANY function f(x), odd or even, can be written in terms of these stationary states). For example, you definatelly cannot write f(x)=cos(x) using only a basis of sine functions. Is there something I am missing here??? Thanks!