Halliday & Matthiessen (2004: 571, 572):
The most direct form of lexical cohesion is the repetition of a lexical item; … in order for a lexical item to be recognised as repeated it need not be in the same morphological shape. … Inflexional variants always belong together as one item; derivational variants usually do, when they are based on a living derivational process, although these are less predictable.