ok, this isn't quite a sine wave, but rather, as one of the commenters in the youtube video i got this from astutely called it, a potato. there is still a fair bit of THD, but its still pretty neat: you take a triangle, square it, invert, add dc offset, and then square again
compared to the primary 'parabol' oscillator, it has even and odd order harmonics instead of only odd, but has a somewhat steeper roll-off. anyway, i just like neat waveform tricks like these. it reminds me of how you can integrate a saw wave by squaring it. if anyone knows of any other tricks like these, feel free to share!