lavinia said:
As an abelian group under addition (not multiplication) ##R## is the same as ##Z×Z##. It is a free abelian group on two generators. Mod p how many residue classes are there?
(For perfect rigor you want to prove that ##R## actually is isomorphic to ##Z×Z## as an abelian group.)
-------------------------------------------------------------------------------------------------------------------------------------------------------Hi Lavinia, fresh_42
Thanks for the hint, Lavinia ... will proceed as far as I can ...
... ...
To show that ##R \cong \mathbb{Z} \times \mathbb{Z}## ... ...Let ##r, t \in R## where ##r = a + b \sqrt{5} i## and ##t = c + d \sqrt{5} i##
Define ##\phi## as follows ...
##\phi \ : \ R \longrightarrow \mathbb{Z} \times \mathbb{Z}## is defined such that:
##\phi (r) = \phi (a + b \sqrt{5} i ) = (a,b)##
then
##\phi (r + s) = \phi ( (a + b \sqrt{5} i) + ( c + d \sqrt{5} i )##
##= \phi ( ( a+c ) + (b + d) \sqrt{5} i )##
##= (a+c, b + d) = (a, b) + (c,d)##
##= \phi ( r) + \phi (t) ##... so ##\phi## is an additive group homomorphism ... ... clearly it is also injective and surjective ...
... so ##\phi## is an isomorphism between ##R## and ##\mathbb{Z} \times \mathbb{Z}## viewed as additive abelian groups ...
... that is ##R \ \cong \ \mathbb{Z} \times \mathbb{Z}##
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Now we have (see previous post ... )
##p = p + 0. \sqrt{5} i = u^2 + 5 v^2## ( see previous post where ##N(s) = p = u^2 + 5 v^2## )... ... now we have ##R/ <p> \ \cong \ \mathbb{Z} \times \mathbb{Z}/ < \phi(p) > ## ... ... (BUT ... is this the case ... most unsure ...? !)... ... now ##\phi (p) = (p,0)## ...... ... so consider ##\mathbb{Z} \times \mathbb{Z} / < \phi (p) > \ = \ \mathbb{Z} \times \mathbb{Z} / < (p, 0 ) >##
BUT...
... this seems to imply there are ##p## residue classes ... namely ##(0,0) , (1,0) , (2,0) , \ ... \ ... \ (p-1, 0)## ...
... seems like something is wrong ...
Can you help further ... seems like I should be working with ##\mathbb{Z} \times \mathbb{Z} / < (p, p ) >## ... ... but why ... ?
I am also very unsure of what ##R \cong \mathbb{Z} \times \mathbb{Z}## implies for the relationship between ##R/ <p>## and ##\mathbb{Z} \times \mathbb{Z} / < \phi (p) >## ...Hope you can help ...
Peter