Date | November Example questions | Marks available | 3 | Reference code | EXN.3.AHL.TZ0.2 |
Level | Additional Higher Level | Paper | Paper 3 | Time zone | Time zone 0 |
Command term | Show that | Question number | 2 | Adapted from | N/A |
Question
A Gaussian integer is a complex number, , such that where . In this question, you are asked to investigate certain divisibility properties of Gaussian integers.
Consider two Gaussian integers, and , such that for some Gaussian integer .
Now consider two Gaussian integers, and .
The norm of a complex number , denoted by , is defined by . For example, if then .
A Gaussian prime is a Gaussian integer, , that cannot be expressed in the form where are Gaussian integers with .
The positive integer is a prime number, however it is not a Gaussian prime.
Let be Gaussian integers.
The result from part (h) provides a way of determining whether a Gaussian integer is a Gaussian prime.
Find .
Determine whether is a Gaussian integer.
On an Argand diagram, plot and label all Gaussian integers that have a norm less than .
Given that where , show that .
By expressing the positive integer as a product of two Gaussian integers each of norm , show that is not a Gaussian prime.
Verify that is not a Gaussian prime.
Write down another prime number of the form that is not a Gaussian prime and express it as a product of two Gaussian integers.
Show that .
Hence show that is a Gaussian prime.
Use proof by contradiction to prove that a prime number, , that is not of the form is a Gaussian prime.
Markscheme
* This sample question was produced by experienced DP mathematics senior examiners to aid teachers in preparing for external assessment in the new MAA course. There may be minor differences in formatting compared to formal exam papers.
(M1)A1
[2 marks]
(M1)A1
(Since and/or are not integers)
is not a Gaussian integer R1
Note: Award R1 for correct conclusion from their answer.
[3 marks]
plotted and labelled A1
plotted and labelled A1
Note: Award A1A0 if extra points to the above are plotted and labelled.
[2 marks]
(and as ) A1
then AG
[1 mark]
A1
and R1
(since are positive) R1
so is not a Gaussian prime, by definition AG
[3 marks]
(A1)
A1
so is not a Gaussian prime AG
[2 marks]
For example, (M1)A1
[2 marks]
METHOD 1
Let and
LHS:
M1
A1
A1
A1
RHS:
M1
A1
LHS = RHS and so AG
METHOD 2
Let and
LHS
M1
A1
M1A1
A1
A1
(= RHS) AG
[6 marks]
which is a prime (in ) R1
if then R1
we cannot have R1
Note: Award R1 for stating that is not the product of Gaussian integers of smaller norm because no such norms divide
so is a Gaussian prime AG
[3 marks]
Assume is not a Gaussian prime
where are Gaussian integers and M1
M1
A1
It cannot be from definition of Gaussian prime R1
hence R1
If then which is a contradiction R1
hence a prime number, , that is not of the form is a Gaussian prime AG
[6 marks]