 A stationary point \(a\) in \(\bR\) of a function \(f\): \(\bR \rightarrow \bR\) is defined to be one in whose vicinity “infinitesimal variations” fail to change the value of \(f\), that is, such that \(f(a + \varepsilon) = f(a)\) for all \(\varepsilon\). This means that \(f(a) + \varepsilon f '(a)= f(a)\), so that \(\varepsilon f'(a) = 0\) for all \(\varepsilon\), whence it follows from microcancellation that \(f'(a) = 0\). This is Fermat’s rule.