Smaller class invariants for constructing curves of genus 2 Marco - PowerPoint PPT Presentation

Smaller class invariants for constructing curves of genus 2 Marco Streng Geocrypt Corsica June 2011

Overview genus 1 genus 2 constructing curves part 1 part 2 smaller class invariants part 3 part 4

Part 1: The Hilbert class polynomial Definition: The j-invariant is 2 8 3 3 b 3 E : y 2 = x 3 + bx + c . j ( E ) = for 2 2 b 3 + 3 3 c 2 ⇒ E ∼ Fact: j ( E ) = j ( F ) ⇐ = k F Definition: Let K be an imaginary quadratic number field. Its Hilbert class polynomial is � � � H K = X − j ( E ) ∈ Z [ X ] . E / C End( E ) ∼ = O K Application 1: roots generate Hilbert class field of K Application 2: elliptic curves with prescribed Frobenius

Elliptic curves with prescribed Frobenius Algorithm: (given π ∈ O K imag. quadr. with p = ππ prime) 1. Compute H K mod p , it splits into linear factors. 2. Let j 0 ∈ F p be a root and let E 0 / F p have j ( E 0 ) = j 0 . 3. Select the twist E of E 0 with “Frob = π ”. It satisfies # E ( F p ) = N ( π − 1) = p + 1 − tr( π ) .

The size ◮ The Hilbert class polynomial of K = Q ( √− 71) is X 7 + 313645809715 X 6 − 3091990138604570 X 5 + 98394038810047812049302 X 4 − 823534263439730779968091389 X 3 + 5138800366453976780323726329446 X 2 − 425319473946139603274605151187659 X + 737707086760731113357714241006081263 . ◮ Weber (around 1900) replaces this by X 7 + X 6 − X 5 − X 4 − X 3 + X 2 + 2 X − 1 .

Part 2: curves of genus 2 “Definition” (char. � = 2): A curve of genus 2 is y 2 = f ( x ) , deg( f ) ∈ { 5 , 6 } , where f has no double roots.

Igusa invariants Igusa gave a genus-2 analogue of the j -invariant, ◮ i.e., a model for the moduli space of genus-2 curves. ◮ Mestre’s algorithm (available in Magma) constructs an equation for the curve from its invariants. ◮ Generically, it suffices to use a triple of absolute Igusa invariants i 1 , i 2 , i 3 ∈ Q ( M 2 ). ◮ See my recent preprint “Computing Igusa class polynomials” on arXiv for the “best” triple.

Complex multiplication Abelian varieties: ◮ An elliptic curve is a 1-dim. ab. var. ◮ The Jacobian of a genus-2 curve is a 2-dim. ab. var. CM-fields: ◮ A CM-field is a field K = K 0 ( √ r ) with K 0 a totally real number field and r ∈ K 0 totally negative. ◮ Let A / C be a g -dim. ab. var. We say that A has CM if there exists O ⊂ End( A ) such that O is an order in a CM-field of degree 2 g . Examples: ◮ g = 1, K 0 = Q , K imaginary quadratic ◮ g = 2, K 0 is real quadratic, K = Q [ X ] / ( X 4 + AX 2 + B )

The reflex CM-type Let K be a CM-field of degree 2 g . It has g pairs of complex conjugate embeddings into C . Definitions: ◮ A CM-type Φ = { φ 1 , . . . , φ g } of K is a choice of one embedding from each pair. ◮ ∃ natural way to associate a CM-type to any CM ab. var. ◮ The type norm of Φ is the map N Φ : x �→ � φ ∈ Φ φ ( x ). Note: N Φ ( x ) N Φ ( x ) = N K / Q ( x ). ◮ The reflex field of Φ is K r = Q ( N Φ ( x ) : x ∈ K ). Example: ◮ If g = 1, then Φ : K → K r identifies K with K r , and N Φ = id. ◮ If K / Q is cyclic quartic, then K r ∼ = K .

Igusa class polynomials Preliminary definition: Let K be a CM field of degree 4. Its Igusa class polynomials are � H i 1 = ( X − i 1 ( C )) ∈ Q [ X ] C � � H i 1 , i n = i n ( C ) ( X − i 1 ( D )) ∈ Q [ X ] ( n ∈ { 2 , 3 } ) D �∼ C = C with products and sums taken over all isom. classes of C / C with CM by O K . Assume: (simplicity only, and true in practice) H i 1 no double roots. i n ( C ) = H i 1 , i n ( i 1 ( C )) Then H i 1 ( i 1 ( C )) = 0 and i 1 ( i 1 ( C )) . H ′

Igusa class polynomials Preliminary definition: Let K be a CM field of degree 4. Its Igusa class polynomials are � ( X − i 1 ( C )) ∈ K r H i 1 = 0 [ X ] C � � ( X − i 1 ( D )) ∈ K r H i 1 , i n = i n ( C ) 0 [ X ] ( n ∈ { 2 , 3 } ) D �∼ C = C with products and sums taken over all isom. classes of C / C with CM by O K of a given CM-type Φ. Assume: (simplicity only, and true in practice) H i 1 no double roots. i n ( C ) = H i 1 , i n ( i 1 ( C )) Then H i 1 ( i 1 ( C )) = 0 and i 1 ( i 1 ( C )) . H ′

Igusa class polynomials Definition: Let K be a CM field of degree 4. Its Igusa class polynomials are � ( X − i 1 ( C )) ∈ K r H i 1 = 0 [ X ] C � � ( X − i 1 ( D )) ∈ K r H i 1 , i n = i n ( C ) 0 [ X ] ( n ∈ { 2 , 3 } ) D �∼ C = C with products and sums taken over one Gal ( K r / K r ) -orbit of isom. classes of C / C with CM by O K of a given CM-type Φ. Assume: (simplicity only, and true in practice) H i 1 no double roots. i n ( C ) = H i 1 , i n ( i 1 ( C )) Then H i 1 ( i 1 ( C )) = 0 and i 1 ( i 1 ( C )) . H ′

Example √ √ √ � K r = Q ( K = Q ( − 14 + 2 5) , ω = 11 , − 7 + 2 ω ) H i 1 = y 4 − 16906968 y 3 + 54245326531032 y 2 + 6990615303516000 y − 494251688841750000 7 4 H i 1 , i 2 = 1181176456752 y 3 − 6134558308934655456 y 2 − 1236449605135697928000 y + 79084224228190734000000 7 4 H i 1 , i 3 = 1782128620567774368 y 3 − 9232752428041223776093632 y 2 − 1189728258050864079984816000 y + 84118511880173912009148000000

Example √ √ √ � K r = Q ( K = Q ( − 14 + 2 5) , ω = 11 , − 7 + 2 ω ) H i 1 = y 2 + (1250964 ω − 8453484) y + 374134464 ω − 1022492484 7 4 H i 1 , i 2 = ( − 139899783096 ω + 590588228376) y − 45253281038112 ω + 143469827584272 7 4 H i 1 , i 3 = ( − 211915358558075664 ω + 891064310283887184) y − 44591718318414329664 ω + 138345299573665361184

The reflex CM-type Definitions: ◮ Recall: a CM-type Φ = { φ 1 , . . . φ g } of K is a choice of one embedding K → C from each complex conjugate pair. ◮ The reflex type Φ r of Φ is the set of those ψ : K r → K that can be extended such that ψ ◦ φ = id K for some φ ∈ Φ. Example: If K is cyclic quartic, then Φ r = { φ − 1 : φ ∈ Φ } Fact: Let K rr be the reflex field of Φ r . Then N Φ r : K r − K rr → ⊂ K � x �− → ψ ( x ) ψ ∈ Φ r

Genus-2 curves with prescribed Frobenius Fix a CM-type Φ and let H ··· be Igusa class polynomials for Φ. Algorithm: (given π ∈ O K quartic CM with p = ππ prime) 1. write ( π ) = N Φ r ( P ) for some P ⊂ O K r 2. compute ( H i 1 mod P ), which splits into linear factors over F p 3. let i 0 1 be a root, let n = H i 1 , i n ( i 0 1 ) i 0 i n ( C 0 ) = i 0 1 ) , and let n ; i 1 ( i 0 H ′ then a twist C of C 0 has “Frob = π ”. It satisfies # J ( C )( F p ) = N ( π − 1) and # C ( F p ) = p + 1 − tr( π ) . Another advantage of our definition: any root i 0 1 is ok (instead of only half of them).

Part 3: back to genus 1 Over C , every elliptic curve is C / Λ. Can choose a Z -basis for Λ and a C -basis for C . Get Λ = τ Z + Z , Im τ > 0. ◮ j is a function of τ , invariant under all changes of bases. ◮ Weber got smaller polynomials by using a more general modular function f , invariant only under some changes of bases.

Modular forms Definition: ◮ For A = ( a b d ) ∈ SL 2 ( Z ), let A τ = a τ + b c τ + d . c ◮ A modular form of weight k and level N is a holomorphic map f : H → C satisfying f ( A τ ) = ( c τ + d ) k f ( τ ) for all A ∈ SL 2 ( Z ) with A ≡ 1 mod N , and a convergence condition at the cusps. n =0 a n q n / N with q = e 2 π i τ . ◮ It has a q-expansion f ( τ ) = � ∞ ∞ � Example: η ( z ) = q 1 / 24 (1 − q n ) for N = 24 , k = 1 / 2 n =1

Modular functions Definition: � g 1 � g i of level N and of equal weight, with Let F N = : coefficients of the q -expansion in Q ( ζ N ) g 2 ◮ recall g i ( A τ ) = ( c τ + d ) k g i ( τ ) if A ≡ 1 mod N ◮ so f ( A τ ) = f ( τ ) if f ∈ F N and A ≡ 1 mod N Conclusion: Action of SL 2 ( Z / N Z ) on F N by f A ( τ ) := f ( A τ ) Examples: ◮ F 1 = Q ( j ) η ( z +1 2 ) ◮ Weber used f ( z ) = ζ − 1 ∈ F 48 , where ζ 48 = e 2 π i / 48 . 48 η ( z )

Class invariants ◮ The Hilbert class field H K of K is the largest unramified abelian extension of K . ◮ K ( j ( τ )) = H K if Z τ + Z has CM by O K . ◮ For f ∈ F N , we call f ( τ ) a class invariant if K ( f ( τ )) = H K . Examples: ◮ j ( τ ) ◮ Weber: if disc( K ) ≡ 1 , 17 mod 24, then for some explicit τ also f ( τ )

Galois groups of modular functions Actions: ◮ SL 2 ( Z / N Z ) acts on F N by f A ( τ ) := f ( A τ ) ◮ Gal( Q ( ζ N ) / Q ) = ( Z / N Z ) ∗ acts on F N by acting on the coefficients of the q -expansion ◮ Let ( Z / N Z ) ∗ ⊂ GL 2 ( Z / N Z ) via v �→ ( 1 0 v ). 0 ◮ Given A ∈ GL 2 ( Z / N Z ), let v = det( A ). Then A = ( 1 0 v )[( 1 0 v ) − 1 A ]. 0 0 In fact: Gal( F N / F 1 ) = GL 2 ( Z / N Z ) / {± 1 }

� Galois groups of values of modular functions ◮ Let τ Z + Z be an O K -module. ◮ The values f ( τ ) as f ranges over F N generate the ray class field H N K of K mod N . ◮ Gal( H N K / H K ) = ( O K / N O K ) ∗ / O ∗ K . τ H N F N K GL 2 ( Z / N Z ) / ± 1 ( O K / N O K ) ∗ / O ∗ K τ � H K Q ( j )

� Galois groups of values of modular functions τ H N F N K ( O K / N O K ) ∗ / O ∗ GL 2 ( Z / N Z ) / ± 1 K τ � H K Q ( j ) Shimura’s reciprocity law: We have f ( τ ) x = f g τ ( x ) ( τ ) for some map g τ : ( O K / N O K ) ∗ → GL 2 ( Z / N Z ) Explicitly: ◮ g τ ( x ) is the transpose of the matrix of multiplication by x w.r.t. the basis τ , 1 of τ Z + Z . ◮ If f is fixed under g τ (( O K / N O K ) ∗ ), then f ( τ ) ∈ H K .