 In the quantum theory the well-definedness of the partition functions of D-branes implies that the RR field strengths obey Dirac quantization conditions when spacetime is compact, or when a spatial slice is compact and one considers only the (magnetic) components of the field strength which lie along the spatial directions.This led twentieth century physicists to classify RR field strengths using cohomology with integral coefficients.However some authors have argued that the cohomology of spacetime with integral coefficients is too big.