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Distribution of Rational Points on Toric Varieties: a Multi-Height Approach

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Distribution of Rational Points on Toric Varieties: a Multi-Height Approach Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Arda H. Demirhan University of Illinois at Chicago March 19, 2021 Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Height Machine associates counting devices to divisor classes. ( ) ( ) geometric facts arithmetic facts , given by divisor relations given by height relations Height Machine is useful for other fields too. How to Count Rational Points? X: an algebraic variety in Pn over a number field Main Idea: We use height functions to count rational points. Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Height Machine is useful for other fields too. How to Count Rational Points? X: an algebraic variety in Pn over a number field Main Idea: We use height functions to count rational points. Height Machine associates counting devices to divisor classes. ( ) ( ) geometric facts arithmetic facts , given by divisor relations given by height relations Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach How to Count Rational Points? X: an algebraic variety in Pn over a number field Main Idea: We use height functions to count rational points. Height Machine associates counting devices to divisor classes. ( ) ( ) geometric facts arithmetic facts , given by divisor relations given by height relations Height Machine is useful for other fields too. Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Counting Function: N(U(K); B) = cardfx 2 U(K): HK (x) ≤ Bg Let U ⊆ X be a Zariski open with some rational points. Geometry shapes Arithmetic (for curves) 2 g = 0 N(B) ≈ cB r=2 g = 1 N(B) ≈ c(log(B) ) g > 1 N(B) ≈ c Another important geometric invariant is the canonical class. Counting functions Slogan: Geometry Arithmetic Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Another important geometric invariant is the canonical class. Counting functions Slogan: Geometry Arithmetic Counting Function: N(U(K); B) = cardfx 2 U(K): HK (x) ≤ Bg Let U ⊆ X be a Zariski open with some rational points. Geometry shapes Arithmetic (for curves) 2 g = 0 N(B) ≈ cB r=2 g = 1 N(B) ≈ c(log(B) ) g > 1 N(B) ≈ c Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Counting functions Slogan: Geometry Arithmetic Counting Function: N(U(K); B) = cardfx 2 U(K): HK (x) ≤ Bg Let U ⊆ X be a Zariski open with some rational points. Geometry shapes Arithmetic (for curves) 2 g = 0 N(B) ≈ cB r=2 g = 1 N(B) ≈ c(log(B) ) g > 1 N(B) ≈ c Another important geometric invariant is the canonical class. Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach This is unfortunately false in general: Manin's Conjecture X: Fano variety i.e. −KX is ample. Then Conjecture (Batryev-Manin) X: smooth projective variety. There is a finite extension F of k with X (F ) Zariski dense in X . Moreover, for a small enough open set U N(U(F ); B) = CB(log(B))t−1(1 + o(1)) as B ! 1. Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Manin's Conjecture X: Fano variety i.e. −KX is ample. Then Conjecture (Batryev-Manin) X: smooth projective variety. There is a finite extension F of k with X (F ) Zariski dense in X . Moreover, for a small enough open set U N(U(F ); B) = CB(log(B))t−1(1 + o(1)) as B ! 1. This is unfortunately false in general: Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach 1 Xn+2: Smooth Fano variety n+2 2 A ,! Xn+2 as a Zariski open set 2 3 Pic(X ) =∼ Z p n+2 4 Q( −3) ⊆ k0 a number field 3 N(U(k0); B) ≥ cB(log(B)) A Counter-Example Example n 3 (Batyrev-Tschinkel) Xn+2: hypersurface in P × P given by 3 3 3 3 `0(x)y0 + `1(x)y1 + `2(x)y2 + `3(x)y3 = 0 where n > 2 Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach n+2 2 A ,! Xn+2 as a Zariski open set 2 3 Pic(X ) =∼ Z p n+2 4 Q( −3) ⊆ k0 a number field 3 N(U(k0); B) ≥ cB(log(B)) A Counter-Example Example n 3 (Batyrev-Tschinkel) Xn+2: hypersurface in P × P given by 3 3 3 3 `0(x)y0 + `1(x)y1 + `2(x)y2 + `3(x)y3 = 0 where n > 2 1 Xn+2: Smooth Fano variety Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach 2 3 Pic(X ) =∼ Z p n+2 4 Q( −3) ⊆ k0 a number field 3 N(U(k0); B) ≥ cB(log(B)) A Counter-Example Example n 3 (Batyrev-Tschinkel) Xn+2: hypersurface in P × P given by 3 3 3 3 `0(x)y0 + `1(x)y1 + `2(x)y2 + `3(x)y3 = 0 where n > 2 1 Xn+2: Smooth Fano variety n+2 2 A ,! Xn+2 as a Zariski open set Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach p 4 Q( −3) ⊆ k0 a number field 3 N(U(k0); B) ≥ cB(log(B)) A Counter-Example Example n 3 (Batyrev-Tschinkel) Xn+2: hypersurface in P × P given by 3 3 3 3 `0(x)y0 + `1(x)y1 + `2(x)y2 + `3(x)y3 = 0 where n > 2 1 Xn+2: Smooth Fano variety n+2 2 A ,! Xn+2 as a Zariski open set ∼ 2 3 Pic(Xn+2) = Z Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach A Counter-Example Example n 3 (Batyrev-Tschinkel) Xn+2: hypersurface in P × P given by 3 3 3 3 `0(x)y0 + `1(x)y1 + `2(x)y2 + `3(x)y3 = 0 where n > 2 1 Xn+2: Smooth Fano variety n+2 2 A ,! Xn+2 as a Zariski open set 2 3 Pic(X ) =∼ Z p n+2 4 Q( −3) ⊆ k0 a number field 3 N(U(k0); B) ≥ cB(log(B)) Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Toric Varieties Some equivariant compactifications Flag varieties Smooth complete intersections of small degree Some Good News Some Examples MC is true for some classes of varieties including: Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Some Good News Some Examples MC is true for some classes of varieties including: Toric Varieties Some equivariant compactifications Flag varieties Smooth complete intersections of small degree Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Abscissa of Convergence: β = βU (L) := inf fσ : ZU (L; s) converges for <s > σg The Connection with the growth function: log N(L; U; B) β = lim sup B!1 log B Accumulating subvarieties: X ⊂ U is accumulating when βU (L) = βX (L) > βUnX (L) Accumulating Subvarieties Let U be a countable set, hL : U ! R+ a function such that its growth is finite for all bounds. Zeta function: X −s ZU (L; s) = hL(x) x2U Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach The Connection with the growth function: log N(L; U; B) β = lim sup B!1 log B Accumulating subvarieties: X ⊂ U is accumulating when βU (L) = βX (L) > βUnX (L) Accumulating Subvarieties Let U be a countable set, hL : U ! R+ a function such that its growth is finite for all bounds. Zeta function: X −s ZU (L; s) = hL(x) x2U Abscissa of Convergence: β = βU (L) := inf fσ : ZU (L; s) converges for <s > σg Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach Accumulating Subvarieties Let U be a countable set, hL : U ! R+ a function such that its growth is finite for all bounds. Zeta function: X −s ZU (L; s) = hL(x) x2U Abscissa of Convergence: β = βU (L) := inf fσ : ZU (L; s) converges for <s > σg The Connection with the growth function: log N(L; U; B) β = lim sup B!1 log B Accumulating subvarieties: X ⊂ U is accumulating when βU (L) = βX (L) > βUnX (L) Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach xm+1 = ··· = xn = 0 n V : blow up of P along L0 n n−m−1 Projections: π1 : V ! P and π2 : V ! P The Exceptional divisor E and the hyperplane section L of π1 satisfy: 1 π1 is an isomorphism away from E. 2 The Picard group of V , Pic(V ) = Z[L] ⊕ Z[E]. 3 The canonical class, KV = −(n + 1)L + (n − m − 1)E. 4 Ample divisors D are of the form D = aL − bE for a > b > 0. Blown up Projective Space Blowing up Pn along Pm n 0 ≤ m ≤ n − 2 and L0: linear subspace of P defined by Arda H. Demirhan Distribution of Rational Points on Toric Varieties: A Multi-Height Approach n n−m−1 Projections: π1 : V ! P and π2 : V ! P The Exceptional divisor E and the hyperplane section L of π1 satisfy: 1 π1 is an isomorphism away from E. 2 The Picard group of V , Pic(V ) = Z[L] ⊕ Z[E]. 3 The canonical class, KV = −(n + 1)L + (n − m − 1)E.
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