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<p>of a four-vector with respect to a scalar  (invariant) is itself a four-vector. It is also useful to take the <a href="page.php?w=differential_of_a_function">differential</a> of the four-vector,  and divide it by the differential of the scalar, :</p>

<p>where the contravariant components are:</p>

<p>while the covariant components are:</p>

<p>In relativistic mechanics, one often takes the differential of a four-vector and divides by the differential in <a href="page.php?w=proper_time">proper time</a> (see below).</p>

<p><big> Fundamental four-vectors </big></p><p>
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