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Certain exact relations among transverse momentum dependent partondistribution functions due to QCD equations of motion turn into approximateones upon the neglect of pure twist-3 terms. On the basis of available datafrom HERMES we test the practical usefulness of one such``Wandzura-Wilczek-type approximation'', namely of that connectingh_{1L}^{\perp(1)a}(x) to h_L^a(x), and discuss how it can be further tested byfuture CLAS and COMPASS data.

Are there approximate relations among transverse momentum dependent distribution functions?