Stumpff function

In celestial mechanics, the Stumpffff functions c_k(x), developed by Karl Stumpff, are used for analyzing orbits using the universal variable formulation. They are defined by the formula:

$c_{k}(x)={\frac {1}{k!}}-{\frac {x}{(k+2)!}}+{\frac {x^{2}}{(k+4)!}}-...=\sum _{i=0}^{\infty }{\frac {(-1)^{i}x^{i}}{(k+2i)!}}$

for $k=0,1,2,3....$ The series above converges absolutely for all real x.

By comparing the Taylor series expansion of the trigonometric functions sin and cos with c₀(x) and c₁(x), a relationship can be found:

$c_{0}(x)=\cos {\sqrt {x}}$, for $x>0$

$c_{1}(x)={\frac {\sin {\sqrt {x}}}{\sqrt {x}}}$, for $x>0$

Similarly, by comparing with the expansion of the hyperbolic functions sinh and cosh we find:

$c_{0}(x)=\cosh {\sqrt {-x}}$, for $x<0$

$c_{1}(x)={\frac {\sinh {\sqrt {-x}}}{\sqrt {-x}}}$, for $x<0$

The Stumpff functions satisfy the recursive relations:

$xc_{k+2}(x)={\frac {1}{k!}}-c_{k}(x)$, for $k=0,1,2,....$