Math 149. Capstone Mathematics for Teachers. Spring 2012.
Homework 8
Subfields of the field of real numbers
Let Q denote the set of rational numbers.
- Let Q(√ 2 ) = { a + b*√ 2 | a and b are
in Q}. Show that this set, with the usual addition and multiplication, is a field.
- Show that √ 3 is not an element of Q(√ 2 ).
- Similarly to Q(√ 2 ), we can define
Q(√ 3 ) =
{ a + b*√ 3 | a and b are in Q} and show that it
is a field (you do not have to write our this proof; it is very similar to the proof for Q(√ 2 ).
Recall that if F1 and F2 are fields, a function f : F1 -> F2 is a field isomorphism if it is one-to-one and onto, and
f(a+b)=f(a)+f(b) and f(ab)=f(a)f(b) for all a and b in F1.
Consider the function f : Q(√ 2 ) -> Q(√ 3 )
given by f(a+b*√ 2 ) = a+b*√ 3 . Show that f is not a
field isomorphism.
- Show that Q(√ 2 ) and Q(√ 3 ) are not
isomorphic as fields (i.e. that there is no field isomorphism
between these two fields).
Hint: assume there exists a field isomorphism f : Q(√ 2 ) -> Q(√ 3 ), let f(√ 2 ) = x+y*√ 3 . Consider f(√ 2 *√ 2 ) and derive a contradiction.
This page was last revised on 13 March 2012.