N-times differentiable function should go with hyphen? [duplicate]

Suppose that I would like to say "Let f(x) be an N-times differentiable function."

Should it be "N-times" or "N times", and why?

No hyphen seems to be much more common in math, as this Google Books search for "N-times differentiable function" indicates, with six out of six examples hyphenless. (Looking at more pages in the search I see one example with a hyphen and dozens without.) If I were writing this, my instinct would be to use a hyphen.

This may have to do with how we modify times:

"...and will meet with n times as many ethereal particles , and will impinge upon each particle with n times greater velocity" ref

...the lifting force becomes n times greater than it was... ref

I think we agree that a hyphen is needed for a six-time Olympic champion.

However,

... making her a four times champion of the world. ref (1918)

and

...he was not destined to join Harry Vardon and Braid as four-times champion ref (1908)