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Factorials apply only to nonnegative integers but can be extended to complex numbers with positive real parts using the Gamma function, \(\Gamma(n) = \int_0^\infty e^{-x}x^{n-1}dx\).

Newton's mutual attraction force for masses \(m_1\), \(m_2\) is \(F=G\frac{m_1 m_2}{d^2}\), where \(d\) is the distance between them and \(G\) is the gravitational constant.

The \(k\)th Fibonacci number can be written in closed form as \(F(k) = \frac{\varphi^k - (-\frac{1}{\varphi})^k}{\sqrt{5}}\), where \(\varphi\) is the celebrated golden ratio.