What is the difference between でなくand ではなく？

Question

This is the sentence I have just read: >外国へ行くとしたら、ただの旅行ではなく、勉強を 目的として 行きたい。 If I ever have a chance to go abroad then I would like to go to study rather than just travel. The は　feels intuitively correct but what purpose does it serve? There are a number of expressions without the は: >～だけでなく…によっても広められている be popularized not only by ~ but also by >～が必要でなくとも although not in need of >[愛]{あい}でなくてなんだろう if it is not love, what is it? (possibly 愛ではなく、なんだろう？） I can't explain why は　is/is not necessary in these expressions (apart from "these are 決まり文句" but I wanted to get a better understanding than that if possible). I should be very grateful for any insights.

Answer 1

The difference is formality and "proper, grammatical" language. Using ではなく will be a little more formal than でなく. For example, you would use ではなく with your superiors and でなく with your friends.