Which of the following is correct grammatically?
- He has a real genius in mathematics.
- He has a real genius for mathematics.
- He has a real genius at mathematics.
Which of the following is correct grammatically?
'He has a real genius for mathematics'
would be correct grammatically. Having a genius for something indicates having a flair or skill for something. Read more here.
"in" would not be valid with "has".
If you are referring to a person, following statement could be used,
He is a real genius in mathematics.
The word genius can be used as noun or adjective.
To use ‘genius’ as a noun, you have to use the word as if it is a ‘thing’
He is a football genius.
That was a stroke of genius.
To use ‘genius’ as an adjective, you have to use the word as if you were describing something.
He comes up with some amazing, genius ideas!