In mathematics, when referring to pure numbers is largest or biggest correct?

Solution 1:

Between largest and biggest, that is not considering possible other alternatives, one can say that

• largest is formal
• biggest is informal

One does not generally use biggest in formal mathematical text or for that matter, in any formal English text for the most part.

[EDIT]
Then I found,
Rachel Wicaksono on BBC's Learning English

quantity words …
'large', more often than 'big', is used with the following quantity words: 'a large amount', 'on a large scale', 'a large number of', 'a large quantity of', 'a large proportion', 'to a large extent', 'a large percentage of', 'a large part of', 'a large volume' and 'a large area'

and,
- Big vs Large vs Great
- Difference Between Big and Large

Solution 2:

In mathematics and computer science, we often use greatest in reference to the greater than relationship. When referring to a set or range, maximum or upper bound are also used. Biggest and largest are both acceptable for non-technical English, although in a mathematical context they may be confused with the greatest magnitude (absolute value).

There are corresponding antonyms: least, minimum, or lower bound for the smallest value in a set, based on the less than relationship.