From owner-cse191-sp08-list@LISTSERV.BUFFALO.EDU  Thu Feb 28 14:06:14 2008
Date:         Thu, 28 Feb 2008 14:05:31 -0500
From: "William J. Rapaport" <rapaport@cse.Buffalo.EDU>
Subject: 191: Question about sets.
To: CSE191-SP08-LIST@LISTSERV.BUFFALO.EDU

A student writes:

| I was trying to do the virtual homework you gave the class, and was 
| trying to understand the last question (#48 in section 2.2). I was 
| wondering if you could go over sets eventually in class. I know that 
| you'll try to as tomorrow we continue reviewing for the midterm, but 
| I thought that I'd add in this request. 

This question concerns the material we covered in lecture yesterday. To
help you do this one, you should first try problem 49, whose answer is
in the back of the book.

Here, however, is a hint on 48a:

In that problem, Ai = {i, i+1, i+2, ...}, for any i.

So: A1 = {1,2,3,...}
    A2 = {2,3,4,...}
    A3 = {3,4,5,...}, etc.

You are asked to compute UiAi (using the notation I introduced
yesterday, which is easier to type than trying to use subscripts
and superscripts).

So, you need to compute A1 U A2 U A3 U ..., which =
{1,2,3,...} U {2,3,4,...} U {3,4,5,...} U ...

I'll let you take it from there.

From owner-cse191-sp08-list@LISTSERV.BUFFALO.EDU  Sun Mar  2 19:52:38 2008
Date:         Sun, 2 Mar 2008 19:52:23 -0500
From: "William J. Rapaport" <rapaport@cse.Buffalo.EDU>
Subject: 191: Correction to HW #6
To: CSE191-SP08-LIST@LISTSERV.BUFFALO.EDU

| Date: Sun, 02 Mar 2008 12:58:25 -0500
| From: Matt Heavner <mheavner@buffalo.edu>
| To: "William J. Rapaport" <rapaport@cse.Buffalo.EDU>
| Subject: CSE191 Virtual HW Answer Correction?
| 
| Professor Rapaport,
| 
| For p.130 #4d:
| 
| Should the answer to B-A = {f,g,h} rather than {g,h}?
| 
| Thanks,
|    Matt
| 

YES!  Glad you caught it!  Thanks.

From owner-cse191-sp08-list@LISTSERV.BUFFALO.EDU  Sun Mar  2 21:38:09 2008
Date:         Sun, 2 Mar 2008 21:34:25 -0500
From: "William J. Rapaport" <rapaport@cse.Buffalo.EDU>
Subject: 191: Yet another HW 6 correction :-(
To: CSE191-SP08-LIST@LISTSERV.BUFFALO.EDU

------------------------------------------------------------------------
Subject: 191: Yet another HW 6 correction :-(
------------------------------------------------------------------------

| From: <kwhake@buffalo.edu>
| To: "William J. Rapaport" <rapaport@cse.Buffalo.EDU>
| Subject: 191: HW #6
| Date: Sun, 02 Mar 2008 20:50:59 -0500
| 
| Hi,
| 
| For the last problem on HW 6, p. 131: #48 on part b, shouldn't the intersection be {0}?
| 

Aargh!  Yes!  Sorry!

| One more question regarding that problem...  Could you say that the intersections in either part a or b include infinity? 

No, because "infinity" isn't a member of any of those sets.