= Quark number sum rule
{title2=$\int_0^1[f_f(x)-\bar f_f(x)]dx=N_f$}
The <parton distribution function> is a number density. Integrating the quark distribution minus the antiquark distribution counts the net flavour charge of the <hadron>, since every sea quark-antiquark pair contributes zero. The corresponding flavour-vector <conserved current> counts particles minus antiparticles. For a <proton>, the net up/down numbers are two and one; for a <neutron> they are one and two. The rule fixes the difference of distributions, not their sum, and does not imply absence of a sea.
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