N=39 |det|=6.321982650572251100739224469504e30 circ=--+-+++-+++--++++-+-++-+----+---+-+++++ grle= -2.92600708563298503e+22 Tomas Rokicki also found this, and his exact determinant was 6321982650569688652881686691840 which is 2562447857537777664 smaller than my computer said, thus exactly revealing the extent of roundoff error. My computer thus got only 12 significant figures correct.