# Example 3 -- Klein configuration of points

In this example, we identify the Betti characters of the defining ideal of the Klein configuration of points in the projective plane and its square. The defining ideal of the Klein configuration is explicitly constructed in Proposition 7.3 of T. Bauer, S. Di Rocco, B. Harbourne, J. Huizenga, A. Seceleanu, T. Szemberg - Negative Curves on Symmetric Blowups of the Projective Plane, Resurgences, and Waldschmidt Constants. We start by constructing the ideal, its square, and both their resolutions and Betti tables. In order to later use characters, we work over the cyclotomic field obtained by adjoining a primitive 7th root of unity to $\mathbb{Q}$. (This example was precompiled by the package author.)

 i1 : kk=toField(QQ[a]/ideal(sum apply(7,i->a^i))) o1 = kk o1 : PolynomialRing i2 : R=kk[x,y,z] o2 = R o2 : PolynomialRing i3 : f4=x^3*y+y^3*z+z^3*x 3 3 3 o3 = x y + y z + x*z o3 : R i4 : H=jacobian transpose jacobian f4 o4 = {-3} | 6xy 3x2 3z2 | {-3} | 3x2 6yz 3y2 | {-3} | 3z2 3y2 6xz | 3 3 o4 : Matrix R <--- R i5 : f6=-1/54*det(H) 5 5 2 2 2 5 o5 = x*y + x z - 5x y z + y*z o5 : R i6 : I=minors(2,jacobian matrix{{f4,f6}}) 3 5 7 7 4 2 2 4 3 2 5 8 7 o6 = ideal (14x y - 5x z - 3y z - 35x y z + 35x*y z - 7x y*z + z , 3x y - ------------------------------------------------------------------------ 8 4 3 5 2 5 3 2 2 4 7 8 7 5 2 y - 35x y z + 7x*y z - 14x z + 35x y z + 5y*z , x - 5x*y - 7x y z ------------------------------------------------------------------------ 2 4 2 3 4 3 5 7 - 35x y z + 35x y*z + 14y z - 3x*z ) o6 : Ideal of R i7 : RI=res I 1 3 2 o7 = R <-- R <-- R <-- 0 0 1 2 3 o7 : ChainComplex i8 : betti RI 0 1 2 o8 = total: 1 3 2 0: 1 . . 1: . . . 2: . . . 3: . . . 4: . . . 5: . . . 6: . . . 7: . 3 . 8: . . . 9: . . 1 10: . . . 11: . . 1 o8 : BettiTally i9 : I2=I^2; o9 : Ideal of R i10 : RI2=res I2 1 6 6 1 o10 = R <-- R <-- R <-- R <-- 0 0 1 2 3 4 o10 : ChainComplex i11 : betti RI2 0 1 2 3 o11 = total: 1 6 6 1 0: 1 . . . 1: . . . . 2: . . . . 3: . . . . 4: . . . . 5: . . . . 6: . . . . 7: . . . . 8: . . . . 9: . . . . 10: . . . . 11: . . . . 12: . . . . 13: . . . . 14: . . . . 15: . 6 . . 16: . . . . 17: . . 3 . 18: . . . . 19: . . 3 . 20: . . . . 21: . . . 1 o11 : BettiTally

The unique simple group of order 168 acts as described in §2.2 of BDHHSS. In particular, the group is generated by the elements g of order 7, h of order 3, and i of order 2, and is minimally defined over the 7th cyclotomic field. In addition, we consider the identity, the inverse of g, and another element j of order 4 as representatives of the conjugacy classes of the group. The action of the group on the resolution of both ideals is described in the second proof of Proposition 8.1.

 i12 : g=matrix{{a^4,0,0},{0,a^2,0},{0,0,a}} o12 = | a4 0 0 | | 0 a2 0 | | 0 0 a | 3 3 o12 : Matrix kk <--- kk i13 : h=matrix{{0,1,0},{0,0,1},{1,0,0}} o13 = | 0 1 0 | | 0 0 1 | | 1 0 0 | 3 3 o13 : Matrix ZZ <--- ZZ i14 : i=(2*a^4+2*a^2+2*a+1)/7 * matrix{ {a-a^6,a^2-a^5,a^4-a^3}, {a^2-a^5,a^4-a^3,a-a^6}, {a^4-a^3,a-a^6,a^2-a^5} } o14 = | 3/7a5+1/7a4+1/7a3+3/7a2-1/7 -1/7a5+2/7a4+2/7a3-1/7a2-2/7 | -1/7a5+2/7a4+2/7a3-1/7a2-2/7 -2/7a5-3/7a4-3/7a3-2/7a2-4/7 | -2/7a5-3/7a4-3/7a3-2/7a2-4/7 3/7a5+1/7a4+1/7a3+3/7a2-1/7 ----------------------------------------------------------------------- -2/7a5-3/7a4-3/7a3-2/7a2-4/7 | 3/7a5+1/7a4+1/7a3+3/7a2-1/7 | -1/7a5+2/7a4+2/7a3-1/7a2-2/7 | 3 3 o14 : Matrix kk <--- kk i15 : j=-1/(2*a^4+2*a^2+2*a+1) * matrix{ {a^5-a^4,1-a^5,1-a^3}, {1-a^5,a^6-a^2,1-a^6}, {1-a^3,1-a^6,a^3-a} } o15 = | -1/7a5-1/7a4+2/7a2-2/7a+2/7 1/7a5+4/7a4+2/7a3+2/7a2+4/7a+1/7 | 1/7a5+4/7a4+2/7a3+2/7a2+4/7a+1/7 1/7a5-1/7a4+1/7a3+3/7a+3/7 | -2/7a5-1/7a3+2/7a2+2/7a-1/7 1/7a5+3/7a4-1/7a3+3/7a2+1/7a ----------------------------------------------------------------------- -2/7a5-1/7a3+2/7a2+2/7a-1/7 | 1/7a5+3/7a4-1/7a3+3/7a2+1/7a | 2/7a4-1/7a3-2/7a2-1/7a+2/7 | 3 3 o15 : Matrix kk <--- kk i16 : G={id_(R^3),i,h,j,g,inverse g}; i17 : A1=action(RI,G,Sub=>false) o17 = ChainComplex with 6 actors o17 : ActionOnComplex i18 : A2=action(RI2,G,Sub=>false) o18 = ChainComplex with 6 actors o18 : ActionOnComplex i19 : elapsedTime character A1 -- 2.41291 seconds elapsed o19 = HashTable{0 => GradedCharacter{{0} => Character{1, 1, 1, 1, 1, 1}} } 4 2 4 2 1 => GradedCharacter{{8} => Character{3, -1, 0, 1, a + a + a, - a - a - a - 1}} 2 => GradedCharacter{{11} => Character{1, 1, 1, 1, 1, 1}} {13} => Character{1, 1, 1, 1, 1, 1} o19 : HashTable i20 : elapsedTime character A2 -- 91.6164 seconds elapsed o20 = HashTable{0 => GradedCharacter{{0} => Character{1, 1, 1, 1, 1, 1}} } 1 => GradedCharacter{{16} => Character{6, 2, 0, 0, -1, -1}} 4 2 4 2 2 => GradedCharacter{{19} => Character{3, -1, 0, 1, a + a + a, - a - a - a - 1}} 4 2 4 2 {21} => Character{3, -1, 0, 1, a + a + a, - a - a - a - 1} 3 => GradedCharacter{{24} => Character{1, 1, 1, 1, 1, 1}} o20 : HashTable

The character of the resolution of the ideal shows the free module in homological degree two is a direct sum of two trivial representations. It follows that its second exterior power is also trivial. As observed in the second proof of Proposition 8.1 in BDHHSS, the free module in homological degree 3 in the resolution of the square of the ideal is exactly this second exterior power and a trivial representation.

In alternative, we can compute the symbolic square of the ideal modulo the ordinary square. The component of degree 21 of this quotient matches the generators of the last module in the resolution of the ordinary square in degree 24 (by local duality); in particular, it is a trivial representation. We can verify this directly.

 i21 : needsPackage "SymbolicPowers" o21 = SymbolicPowers o21 : Package i22 : Is2 = symbolicPower(I,2); o22 : Ideal of R i23 : M = Is2 / I2; i24 : B = action(M,G,Sub=>false) o24 = Module with 6 actors o24 : ActionOnGradedModule i25 : elapsedTime character(B,21) -- 44.164 seconds elapsed o25 = GradedCharacter{{21} => Character{1, 1, 1, 1, 1, 1}} o25 : GradedCharacter