Both 'a single' and 'one single' are correct and commonly used phrases in English. They are interchangeable and can be used to emphasize the singularity of an item or person.
One-to-one is used when you talk about transfer or communications. You may use one-to-one when you can identify a source and a destination. For eg., a one-to-one email is one sent from a single person to another, i.e., no ccs or bccs. In maths, a one-to-one mapping maps one element of a set to a unique element in a target set. One-on-one is the correct adjective in your example. See Free ...
Which one is grammatically correct or better? I have two assignments, One of them is done. I have two assignments, One of which is done. I watched a video tutorial that the teacher said the ...
pronouns - "One of them" vs. "One of which" - English Language Learners ...
"Which one is the best" is obviously a question format, so it makes sense that " which one the best is " should be the correct form. This is very good instinct, and you could even argue that the grammar is good, but at best it's unnatural.
When using the word " which " is it necessary to still use " one " after asking a question or do " which " and " which one " have the same meaning? Where do you draw the line on the difference between " which " and " which one " when asking a question that involves more than one answer?
Some people say a dog=one, dogs=ones, the dog=the one=that, and the dogs=the ones=those. It's a rule of thumb, but what I found was that this is not always correct.