Σ (Sigma) and Π (Pi) classes

Σ (Sigma) and Π (Pi) classes are categories in computational complexity theory that describe the difficulty of problems based on their logical structure. Σ classes involve problems where solutions can be verified through a series of existential questions (e.g., "Is there a solution ...?"). Π classes involve problems that are verified through universal questions (e.g., "For all solutions, does this condition hold?"). These classes help classify problems based on how hard they are to solve or verify, with higher levels representing more complex logical combinations. They form part of the polynomial hierarchy, offering a framework to understand problem complexity beyond NP.