Definitely not a complex!

Algebra Level 4

If $k$ is a positive integer, then $\dfrac { { k }^{ 7 } }{ 7 } +\dfrac { { k }^{ 5 } }{ 5 } +\dfrac { { 2k }^{ 3 } }{ 3 } -\dfrac { k }{ 105 }$ is definitely a/an $\text{\_\_\_\_\_\_\_\_\_}$ .