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Truncated Mellin moments: Useful relations and implications for the spin structure function $g_2$

We review our previous studies of truncated Mellin moments of parton distributions. We show in detail the derivation of the evolution equation for double truncated moments. The obtained splitting function has the same rescaled form as in a case of the single truncated moments. We apply the truncated moments formalism to QCD analyses of the spin structure functions of the nucleon, $g_1$ and $g_2$. We generalize the Wandzura-Wilczek relation in terms of the truncated moments and find new sum rules. We derive the DGLAP-like evolution equation for the twist-2 part of $g_2$ and solve it numerically. We find also useful relations between the truncated and untruncated moments.