# What fraction of knots are ribbon knots?

Pardon the naive question. Perhaps there are several different variants of my question, but essentially I am seeking to know whether ribbon knots are vanishingly rare among all knots, or a positive fraction of all knots. I am sure this is known to the cogknotscenti...

If the question needs to be specific:

Q. Among prime knots of crossing number $n$ (OEIS A002863), what is the fraction that are ribbon knots, as $n \to \infty$?

• Surely they are vanishingly rare. The Fox-Milnor condition on the Alexander polynomial (that it factors as $f(t)f(t^{-1})$) should already show that slice knots are rare. I don't know a proof, however. – Dylan Thurston Nov 16 '14 at 3:44
• @DylanThurston: Thanks for pointing me to that nice symmetry condition on ribbon (and slice) knots. – Joseph O'Rourke Nov 16 '14 at 13:49