**Derivation of Axial Tension - Compression Linear Elastic Element -
Displacement Function**

**In this section the method of assuming a displacement function will be used. In this case
the displacement function is exact, in many cases it is an approximation which normally increases the number
of elements needed to achieve a result of acceptable accuracy. If a polynomial displacement function is assumed,
then the order of the polynomial must be linked to the number of degrres of freedom of the element.
**