The Fibonacci numbers are defined by the second order linear recurrence relation:
\(F_{n+1}=F_{n}+F_{n-1}\) (
\(n\geq1\)) with the initial conditions
\(F_{0}=0\) and
\(F_{1}=1\). Similarly, the Lucas numbers are defined by
\(L_{n+1}=L_{n}+L_{n-1}\) (
\(n\geq1\)) with the initial conditions
\(L_{0}=2\) and
\(L_{1}=1\). The characteristic equation of
\(F_{n}\) is
The roots of equation (
8) are
\(\ \alpha=\frac{1+\sqrt{5}}{2}\),
\(\beta=\frac{1-\sqrt{5}}{2}\), and the Binet formula for
\(F_{n}\) is
$$ F_{n}=\left \{ \textstyle\begin{array}{@{}l@{\quad}l} \frac{\alpha^{n}+\alpha^{-n}}{\sqrt{5}},& n\mbox{ odd},\\ \frac{\alpha^{n}-\alpha^{-n}}{\sqrt{5}},& n\mbox{ even}. \end{array}\displaystyle \right . $$
(9)
The positive root of equation (
8),
\(\alpha=\frac{1+\sqrt {5}}{2}\), is called the golden ratio, which has been very attractive for researchers because it occurs ubiquitous such as in nature, art, architecture, and anatomy.
The Fibonacci numbers have many properties, continuous versions, and generalizations [
14‐
20]. Stakhov and Tkachenko [
14] introduced a new class of hyperbolic functions called hyperbolic Fibonacci functions replacing the discrete variable
n in equation (
9) with the continuous variable
x that takes its values from the set of real numbers. Based on an analogy between the Binet formula, (
9), and the classical hyperbolic functions,
$$ \sinh(x)=\frac{e^{x}-e^{-x}}{2} \quad\mbox{and}\quad \cosh(x)=\frac {e^{x}+e^{-x}}{2}, $$
(10)
Stakhov and Rozin [
15] defined the so-called symmetrical hyperbolic Fibonacci functions as follows:
$$ sFs(x)=\frac{\alpha^{x}-\alpha^{-x}}{\sqrt{5}}\quad\mbox{and}\quad cFs(x)=\frac{\alpha^{x}+\alpha^{-x}}{\sqrt{5}}, $$
(11)
where
\(sFs(x)\) and
\(cFs(x)\) denote symmetric hyperbolic Fibonacci sine and cosine functions, respectively. Similarly, a symmetric hyperbolic Fibonacci tangent function can be defined as
$$ tFs(x)=\frac{sFs(x)}{cFs(x)}=\frac{\alpha^{x}-\alpha^{-x}}{\alpha ^{x}+\alpha^{-x}}. $$
(12)
The graphs of the symmetrical hyperbolic Fibonacci functions have a symmetric form and are similar to the graphs of the classical hyperbolic functions. Also, the symmetrical hyperbolic Fibonacci functions
\(sFs(x)\) and
\(cFs(x)\) are increasing on
\((0,+\infty)\). The graphs of the symmetrical hyperbolic Fibonacci functions are given in [
15]. The symmetric hyperbolic Fibonacci functions have properties that are similar to the classical hyperbolic functions. Some of them are [
15]:
$$ cFs(x)=cFs(-x),\qquad sFs(x)=-sFs(-x)\quad\mbox{and}\quad \bigl[ cFs(x) \bigr] ^{2}- \bigl[ sFs(x) \bigr] ^{2}=\frac{4}{5}. $$
Also, the derivative hyperbolic Fibonacci functions are [
15]
$$\begin{aligned}& \bigl[ cFs(x) \bigr] ^{(n)} =\left \{ \textstyle\begin{array}{@{}l@{\quad}l} ( \ln\alpha ) ^{n}sFs(x),&\mbox{for }n\mbox{ odd}, \\ ( \ln\alpha ) ^{n}cFs(x),&\mbox{for }n\mbox{ even}, \end{array}\displaystyle \right .\\& \bigl[ sFs(x) \bigr] ^{(n)} =\left \{ \textstyle\begin{array}{@{}l@{\quad}l} ( \ln\alpha ) ^{n}cFs(x),&\mbox{for } n\mbox{ odd}, \\ ( \ln\alpha ) ^{n}sFs(x),&\mbox{for }n\mbox{ even}. \end{array}\displaystyle \right . \end{aligned}$$
For more information and the generalizations as regards hyperbolic Fibonacci functions, see [
15‐
20] the references cited therein.