In this presentation by Vladimir Voevodsky [1], he mentions a result that there is a formula over the natural numbers with a single free variable such that one can prove that there is no algorithmic method of determining set membership for any natural number.
Can anyone know of a reference to this result?
[1] https://www.youtube.com/watch?v=O45LaFsaqMA @ about 2/3 through the lecture... lost the exact location in the video.