Date | May 2013 | Marks available | 5 | Reference code | 13M.3srg.hl.TZ0.2 |
Level | HL only | Paper | Paper 3 Sets, relations and groups | Time zone | TZ0 |
Command term | Show that | Question number | 2 | Adapted from | N/A |
Question
Consider the set S = {1, 3, 5, 7, 9, 11, 13} under the binary operation multiplication modulo 14 denoted by \({ \times _{14}}\).
Copy and complete the following Cayley table for this binary operation.
Give one reason why \(\{ S,{\text{ }}{ \times _{14}}\} \) is not a group.
Show that a new set G can be formed by removing one of the elements of S such that \(\{ G,{\text{ }}{ \times _{14}}\} \) is a group.
Determine the order of each element of \(\{ G,{\text{ }}{ \times _{14}}\} \).
Find the proper subgroups of \(\{ G,{\text{ }}{ \times _{14}}\} \).
Markscheme
A4
Note: Award A3 for one error, A2 for two errors, A1 for three errors, A0 for four or more errors.
[4 marks]
any valid reason, for example R1
not a Latin square
7 has no inverse
[1 mark]
delete 7 (so that G = {1, 3, 5, 9, 11, 13}) A1
closure – evident from the table A1
associative because multiplication is associative A1
the identity is 1 A1
13 is self-inverse, 3 and 5 form an inverse
pair and 9 and 11 form an inverse pair A1
the four conditions are satisfied so that \(\{ G,{\text{ }}{ \times _{14}}\} \) is a group AG
[5 marks]
A4
Note: Award A3 for one error, A2 for two errors, A1 for three errors, A0 for four or more errors.
[4 marks]
{1}
{1, 13}\(\,\,\,\,\,\){1, 9, 11} A1A1
[2 marks]
Examiners report
There were no problems with parts (a), (b) and (d).
There were no problems with parts (a), (b) and (d).
There were no problems with parts (a), (b) and (d) but in part (c) candidates often failed to state that the set was associative under the operation because multiplication is associative. Likewise they often failed to list the inverses of each element simply stating that the identity was present in each row and column of the Cayley table.
The majority of candidates did not answer part (d) correctly and often simply listed all subsets of order 2 and 3 as subgroups.