f = x1^2+2x2^2+x3^2+2x1x2+2x1x3+4x2x3
= (x1+x2+x3)^2+x2^2+2x2x3
= (x1+x2+x3)^2+(x2+x3)^2 -x3^2
= y1^2+y2^2-y3^2
f 既不正定也不负定.