OEIS/A003828
There seem to be more than 19 solutions by Seiji Tomita.
- euler413.narod.ru
- www.maroon.dti.ne.jp has really huge solutions. Current list
- database.txt on euler.fr
- Robert Gerbicz on uhunt.onlinejudge.org
There seem to be more than 19 solutions by Seiji Tomita.