N on n
The 'expert' group
These nouns refer to someone who knows a lot about something.
David Willis, a specialist on US affairs, looks at the implications for the President at home and abroad.
In the case of authority and expert, the preposition on is sometimes followed by an '-ing' clause or a wh-clause.
authority expert specialist whizz
Since when has he been an authority on what children should watch on TV?
This man is a fighter pilot, but also an expert on defeating air defenses.