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COMPUTING IGUSA CLASS POLYNOMIALS VIA THE
CHINESE REMAINDER THEOREM
¨KIRSTEN EISENTRAGER AND KRISTIN LAUTER
Abstract. We present a new method for computing the Igusa class
polynomials of a primitive quartic CM field. For a primitive quartic
CM field, K, we compute the Igusa class polynomials modulo p for
certain small primes p and then use the Chinese remainder theorem
and a bound on the denominators to construct the class polynomials.
We also provide an algorithm for determining endomorphism rings of
Jacobians of genus 2 curves. Our algorithm can be used to generate
genus 2 curves over a finite field F with a given zeta function.n
1. Introduction
In this paper we present a new method for computing the Igusa class
polynomials of a primitive quartic CM field. Our method generalizes the
algorithm for finding the Hilbert Class polynomial given in [ALV04] to the
genus 2 situation. Given a primitive quartic CM field K, for each small
prime p in a certain set we determine the Igusa class polynomial modulo
p by finding all triples of invariants modulo p for which the corresponding
genus2curvehasCMbyK. The Igusaclass polynomialisthenfoundusing
the Chinese Remainder Theorem and a bound on the denominators of the
coefficients.
Several difficulties arise in the genus 2 situation which are absent in the
elliptic curve case. In this paper we resolve the following issues: the field of
definition of a CM abelian variety, necessary conditions on the small primes
for the algorithm to succeed, and the computation of the endomorphism
ring of the Jacobian of a genus 2 curve in the ordinary case.
The triple of Igusa invariants ([Igu60, Igu62]) of a genus 2 curve can be
calculated in two different ways: from modular functions evaluated on a
period matrix or as invariants of the binary sextic defining the curve. Igusa
showed how the invariants of a binary sextic could be expressed in terms of
Siegel modular forms ([Igu67, p. 848]) (see also [GL04, Section 5.2]). So the
absolute Igusa invariants can be computed as quotients of Siegel modular
Key words and phrases. genus 2 curves, endomorphism rings, Igusa class polynomials,
complex multiplication, Chinese Remainder Theorem.
The first author was partially supported by the National Science Foundation under
agreement No. DMS-0111298. We thank E. Goren, E. Howe, K. Kedlaya, J-P. Serre, P.
Stevenhagen, and T. Yang for helpful discussions.
MSC 11G15, 11G10, 11R37, 14G50.
1¨2 KIRSTEN EISENTRAGER AND KRISTIN LAUTER
forms evaluated on the period matrix associated to an abelian surface with
principal polarization, but this approach requires an exponentially large
amount of precision.
GivenaprimitivequarticCMfieldK,letAbeasystemofrepresentatives
for the set of isomorphism classes of principally polarized abelian varieties
overC having complex multiplication byK. For each abelian varietyA∈A
let (j (A),j (A),j (A)) be the absolute Igusa invariants of A. Then the1 2 3
Igusa class polynomials H , for i= 1,2,3, are defined to bei
Y
H := (X−j (A)).i i
A∈A
It is known ([Shi98]) that roots of these polynomials generate unramified
abelian extensions of the reflex field of K. It is also known that Igusa class
polynomials can be used to generate genus 2 curves with CM by K, and
thus with a given zeta function over a suitable prime field (cf. Section 3).
In this paper we prove the following theorem.
Theorem 1. Given a primitive quartic CM fieldK, the following algorithm
finds the Igusa class polynomials of K :
(1) Produce a collection S of small rational primes p∈S satisfying:
a. p splits completely in K and splits completely into principal ideals in
∗K , the reflex of K.
b. Let B be the set of all primes of bad reduction for the genus 2 curves
with CM by K. Then S∩B =∅.Q
c. p>c, where c is a constant determined in Theorem 3.p∈S
(2) Form the class polynomialsH , H , H modulop for eachp∈S. Let1 2 3
H (X):=H (X) mod p. Theni,p i
Y
H (X) = (X−j (C)),i,p i
C∈Tp
where T is the collection of F -isomorphism classes of genus 2 curves overp p
F whose Jacobian has endomorphism ring isomorphic to O .p K
(3) Chinese Remainder Step. Form H (X) from {H } (i= 1,2,3).i i,p p∈S
Remark1. Condition 1(a) is enough to insure thatp solves a relative norm
equation in K/K , ππ =p, π a Weil number (cf. Proposition 4 below).0
Remark 2. By [GL04], the primes in the set B and in the denominators
of the class polynomials are bounded effectively by a quantity related to the
discriminant ofK. Furthermore, it follows from [Gor97, Theorems 1 and 2]
and the discussion in [GL04, Section 4.1] that condition 1(b) is implied by
condition 1(a).
Remark 3. It follows from the Cebotarev density theorem that the density
of the primes in the setS is inversely proportional to the class number ofK
in the case that K is Galois cyclic. In the non-Galois case, the density is
inversely proportional to the degree of the normal closure of the composite
of K with the Hilbert class field of the reflex of K.6
A CRT ALGORITHM FOR COMPUTING IGUSA CLASS POLYNOMIALS 3
Our algorithm has not been efficiently implemented yet, and we make no
claims about the running time. Our algorithm has the advantage that it
does not require exponentially large amounts of precision of computation.
It was recently brought to our attention that the paper [CMKT00] proposes
a similar algorithm, but they give no proof of the validity of the approach.
Indeed, they fail to impose the conditions necessary to make the algorithm
correct and include many unclear statements.
The proof of Theorem 1 is given in Section 4. Implementation details for
thealgorithmaregiveninSection5. InSection6we show how todetermine
theendomorphismringofanordinaryJacobianofagenus2curve. Section7
gives an example of the computation of a class polynomial modulo a small
prime.
2. Notation.
Throughout this paper, C denotes a smooth, projective, absolutely ir-
reducible curve, and J = J(C) will be its Jacobian variety with identity
element O. The field, K, is always assumed to be a primitive quartic CM
field, with ring of integers O . The real quadratic subfield of K is denotedK
by K , and a generator for the Galois group Gal(K/K ) is denoted by a0 0
∗bar, ω 7→ ω¯. We will write K for the reflex of the quartic CM field K.
For i = 1,2,3 we let H (X) be the Igusa class polynomials of K, and fori
a prime p ∈ S we let H := H mod p. For a field F, F will denote ani,p i
algebraic closure of F. We say that C has CM by K if the endomorphism
ring of J(C) is isomorphic to O .K
3. Generating genus 2 curves with a given zeta function
Our algorithm solves the following problem under certain conditions.
Problem: Given (n,N ,N ), find a genus 2 curve C over the prime field1 2
F such that #C(F ) =N and #C(F ) =N . Given such a C, it follows2n n 1 2n
2that #J(C)(F )=N =(N +N )/2−n.n 21
Given (n, N , N ), it is straightforward to find K, the quartic CM field1 2
such that the curve C has CM by K, by finding the quartic polynomial
2 2satisfied by Frobenius. Write N =n+1−s , and N =n +1+2s −s ,1 1 2 2 1
and solve for s and s . Then K is generated over Q by the polynomial1 2
4 3 2 2t −s t +s t −ns t+n .1 2 1
If s is prime to n, then the Jacobian is ordinary ([How95, p. 2366]).2
Assume that (s ,n)=1. We also restrict to primitive CM fields K. If K is2
aquarticCMfield,thenK isnotprimitiveiffK/QisGaloisandbiquadratic
(Gal(K/Q)=V )([Shi98, p.64]). In the example in Section 7,K is given in4p √
the formK =(i a+b d), witha,b,d∈Z andd and (a,b) square free. In
2 2 2this form the condition is easy to check: K is primitive iffa −b d =k for
some integer k ([KW89, p. 135]). Assume further that K does not contain
a cyclotomic field.¨4 KIRSTEN EISENTRAGER AND KRISTIN LAUTER
Given the data, (n, N , N ), satisfying the assumptions, computeK and1 2
its Igusa class polynomials H , H , H using Theorem 1. From a triple1 2 3
of roots modulo p of H , H , H for a prime p ∈ S, we can construct a1 2 3
genus 2 curve over a finite fieldF whose Jacobian has CM by K using thep
combined algorithms of Mestre ([Mes91]) and Cardona-Quer ([CQ02]). If
the curve does not have the required number of points on the Jacobian, a
twistofthecurvemaybeused. Inthecasewhere4groupordersarepossible
for the pair (n,K) (cf. Section 5.1), a different triple of invariants may be
tried until the desired group order is obtained.
4. Proof of Theorem 1
GivenaprimitivequarticCMfieldK,letAbeasystemofrepresentatives
of the isomorphism classes of principally polarized abelian surfaces over C
withCMbyK. EachelementofAhasafieldofdefinitionk whichisafinite
extension of Q ([Shi98, Prop. 26, p. 96]). For any prime p ∈ S satisfying
the conditions of Theorem 1, the setT was defined in Step 2 of Theorem 1p
as the collection of F -isomorphism classes of genus 2 curves over F withp p
endomorphism ring isomorphic to O . We claim that we have a bijectiveK
correspondence between A and T . Moreover, we claim that reducing thep
Igusa invariants gives the Igusa invariants of the reduction. Taken together,
these can be stated in the form of the following theorem:
Theorem 2. Let K be a primitive quartic CM field and let p ∈ S be a
rational prime that satisfies the conditions of Theorem 1. Then
Y
H (X) = (X−j (C)),i,p i
C∈Tp
where H (X) and T are defined as in Theorem 1.i,p p
Proof. LetA∈A be a principally pol

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