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 \(\text{_________} \).