By comparing the Taylor series expansion of the trigonometric functions sin and cos with c 0 (x) and c 1 (x), a relationship can be found: c 0 ( x ) = cos ⁡ ⁡ x , c 1 ( x ) = sin ⁡ ⁡ x x , for x > 0 Similarly, by comparing with the expansion of the hyperbolic functions sinh and cosh we find: c 0 ( x ) = cosh ⁡ ⁡ − − x , c 1 ( x ) = sinh ⁡ ⁡ − − x − − x , for x < 0