Comment by fsh

Even 21 was only possible by cheating (optimizing away the difficult part using prior knowledge of the results) [1]. Craig Gidney has a blog post that shows the actual quantum circuit for factoring 21 which is far beyond the capabilities of current quantum computers [2].

[1] https://www.nature.com/articles/nature12290

[2] https://algassert.com/post/2500