The fact that sine and cosine functions form a complete orthogonal basis (such as in Fourier series) is a foundational mathematical truism that requires no citation.
The claim states that sine and cosine functions form a complete orthogonal basis for signal representation, which is a standard definition and fundamental theorem in mathematical analysis and signal processing (Fourier analysis). By the prompt instructions, a claim that is trivially true by definition or direct mathematical fact is categorized as COMMON_KNOWLEDGE, requiring no empirical citation or paper assignment.