Actually, many dictionaries specifically label "mathematics" as plural, but concede that the singular form is far more popular in practice.
I tend to find only one use case where I find a plural form simply appears more natural: prefixing it with "the", as in "The mathematics necessary to explain the universe are complex." Replacing the "are" with "is" just does not sound right. Funnily enough, the natural pattern with the North American "math" becomes "The math necessary to explain the universe is complex.".
Damn you, strange collective noun.
