Abstract

On some extensions of Gauss’ work and applications

Highlights

  • For a negative integer D such that D ≡ 0 or 1, let (D) be the set of primitive positive definite binary quadratic forms Q(x, y) = ax2 + bxy + cy2 ∈ [x, y] of discriminant b2 − 4ac = D

  • The modular group SL2( ) (or PSL2( )) acts on the set (D) from the right and defines the proper equivalence ∼ as for some γ. In his celebrated work Disquisitiones Arithmeticae of 1801 [1], Gauss introduced the beautiful law of composition of integral binary quadratic forms

  • Let N be a positive integer, n = N K and P be a subgroup of IK(n) satisfying PK,1(n) ⊆ P ⊆ PK(n)

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Summary

Introduction

The modular group SL2( ) (or PSL2( )) acts on the set (D) from the right and defines the proper equivalence ∼ as In his celebrated work Disquisitiones Arithmeticae of 1801 [1], Gauss introduced the beautiful law of composition of integral binary quadratic forms. Given the order of discriminant D in the imaginary quadratic field K = ( D ), let I( ) be the group of proper fractional -ideals and P( ) be its subgroup of nonzero principal -ideals. We shall develop an algorithm of finding distinct form classes in N(dK)/∼Γ and give a concrete example (Proposition 6.2 and Example 6.3) To this end, we shall apply Shimura’s theory which links the class field theory for imaginary quadratic fields and the theory of modular functions ([7, Chapter 6]).

Extended form class groups as ideal class groups
Class field theory over imaginary quadratic fields
Construction of class invariants
Extended form class groups as Galois groups
Finding representatives of extended form classes

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