minimax equivalence 6 quadratic Tits form 15A30 4 15B33 4 16G20 2 Hasse diagram 2 Kleiner's poset 2 Tits \(P\)-critical poset 2 \(0\)-balanced subposet 2 \(P\)-critical poset 2 critical poset 2 minimax sum 2 one-side and two-side sums 2 principal poset 2 quiver 2 semichain 2 serial poset 2 weak isomorphism 2