ABSTRACT
For a commutative ring A with 1, we denote by A × its group of units, that is A × = { u ∈ A ∣ v u = 1 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_1.tif"/> for some v in A } https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_2.tif"/> . In this chapter, we shall show that the group O K × https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_3.tif"/> of units of a number field K is finitely generated. To motivate, let us take a square-free integer m > 1 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_4.tif"/> . For the sake of simplicity, let m ≡ 2 , 3 mod 4 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_5.tif"/> , because then for K = ℚ ( m ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_6.tif"/> , O K = ℤ [ m ] = ℤ ⊕ ℤ m https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_7.tif"/> . Now u = x + y m ∈ O K × https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_8.tif"/> if and only if the norm N ( u ) = x 2 − m y 2 = ± 1. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003177036/f0af01d0-3a24-4c34-9b44-c23e4a0fe932/content/math5_9.tif"/>
