Write the Word or Phrase That Best Completes Each Statement $$\text { The function given by } f ( x ) = - 3 x ^ { 5 } + \sqrt { 2 } x + \frac { 1 } { 2 } x \text { (is/is not) a polynomial function. }$$

Write the word or phrase that best completes each statement or answers the question.
Provide the missing information.
- $$\text { The function given by } f ( x ) = - 3 x ^ { 5 } + \sqrt { 2 } x + \frac { 1 } { 2 } x \text { (is/is not) a polynomial function. }$$