# Type [210], CY of degree 18 via linkage -- lifting to a 3-fold with components of degrees 11, 6, 1

We construct via linkage an arithmetically Gorenstein 3-fold $X = X_{11} \cup X_1 \cup X_6 \subset \bf{P}^7$, of degree 17, having Betti table of type [210]: For an artinian reduction $A_F$, the quadratic part of the ideal $F^\perp$ is the ideal of a plane and a line, and $F^\perp$ contains pencils of ideals of five points on a conic in the plane and three points on the line, and a 3-dimensional family of ideals of six points in the plane and two points on the line. In particular, $F^\perp$ contains the ideal of the intersection point of the line and the plane in addition to five points in the plane and two points on the line.

We construct $X_{11}$ in a quadric in a P6, $X_{6}$ in a quadric in a P5 and $X_{1}$ in the P4 of intersection of P5 and P6. In the construction the intersection $X\cup X'$ of a component $X$ with the other is an anticanonical divisor on $X$.

$\phantom{WWWW} \begin{matrix} &0&1&2&3&4\\\text{total:}&1&11&20&11&1\\ \text{0:}&1&\text{.}&\text{.}&\text{.}&\text{.}\\ \text{1:}&\text{.}&2&1&\text{.}&\text{.}\\ \text{2:}&\text{.}&9&18&9&\text{.}\\ \text{3:}&\text{.}&\text{.}&1&2&\text{.}\\ \text{4:}&\text{.}&\text{.}&\text{.}&\text{.}&1\\ \end{matrix}$

 i1 : kk = QQ; i2 : U=kk[y0,y1,y2,y3,y4,y5,y6,y7]; i3 : X1 = ideal(y0,y5,y6,y7); -- the component X1, a P3 o3 : Ideal of U i4 : P3a = ideal(y0,y1,y2,y3); --another P3 o4 : Ideal of U i5 : P4 = ideal(y0,y6,y7); --a P4 o5 : Ideal of U i6 : P5 = ideal(y6,y7); --a P5 o6 : Ideal of U i7 : P6 = ideal(y0); -- a P6 o7 : Ideal of U

Construct a $(2,2)$ complete intersection in $X_1$:

 i8 : CI22 = X1 + ideal(random(2,P3a),random(2,P3a)); o8 : Ideal of U

Compute the ideal of the union of $CI_{22}$ and the other $\PP^3$. Then compute a complete intersection of type $(1,2,2,2)$ which contains $\PP^3_a$ and $CI_{22}$.

 i9 : CIP3 = intersect(CI22,P3a); --the union of CI22 and P3a o9 : Ideal of U i10 : CI1222 = trim( ideal(y0) + ideal(random(2,CIP3),random(2,CIP3),random(2,CIP3))); o10 : Ideal of U

We take the residual in this complete intersection of the $\PP^3_a$.

 i11 : X7 = CI1222:P3a; -- the 3-fold X7 linked (1,2,2,2) to PL o11 : Ideal of U i12 : (dim X7, degree X7) o12 = (4, 7) o12 : Sequence

 i13 : QX7 = ideal random(2,X7);--a quadric hypersurface that contains X7 o13 : Ideal of U i14 : Z2a = X1 + QX7;-- quadric that contain X7 intersected with X1 o14 : Ideal of U i15 : X7Z2 = intersect(Z2a,X7);-- the union of Z2 and X7 o15 : Ideal of U

 i16 : Z6 = P4 + QX7 + ideal(random(3,X7Z2));--a complete intersection 2,3 in P4 o16 : Ideal of U i17 : X7Z6 = intersect(X7,Z6); --the union of X7 and the complete intersection 2,3 in P4 o17 : Ideal of U i18 : CI1233 = P6 + QX7 + ideal(random(3,X7Z6),random(3,X7Z6));--complete intersection 1233 containing Z2a and a hyperplane section of X7 o18 : Ideal of U i19 : X11 = CI1233:X7; --a 3-fold of degree 11. o19 : Ideal of U i20 : Z4 = Z6:Z2a;--a Del Pezzo surface in P4 o20 : Ideal of U i21 : Y2 = P5+ideal(random(2,Z4));--a quadric 4-fold in P5 that contain Z4 o21 : Ideal of U i22 : Z2b = X1+Y2;-- another quadric surface in X1 o22 : Ideal of U i23 : Z6b = intersect(Z4,Z2b);-- a complete intersection 2,3 different from Z6a, the union of Z4 and Z2b o23 : Ideal of U i24 : Y3 = P5 + ideal(random(3,Z6b));--a cubic 4-fold in P5 that contain Z6b o24 : Ideal of U i25 : X6 = Y2 + Y3; o25 : Ideal of U i26 : X18 = intersect(X11,X6,X1);--a AG 3-fold of degree 18, with betti table of type 210. o26 : Ideal of U
 i27 : (dim X18, degree X18) o27 = (4, 18) o27 : Sequence i28 : betti res X18 0 1 2 3 4 o28 = total: 1 11 20 11 1 0: 1 . . . . 1: . 2 1 . . 2: . 9 18 9 . 3: . . 1 2 . 4: . . . . 1 o28 : BettiTally