Wikipedia:Reference desk/Archives/Mathematics/2015 January 5

= January 5 =

Reverse Polish Notation
Hi all,

I'm trying to understand the following reverse polish notation:

SSP xor DS xor or xor not

If I had the following:

SSP xor DS xor or not

then I know it would be:

NOT((S xor S xor P) or (D xor S))

What would SSP xor DS xor or xor not be?

Letsbefiends (talk) 01:44, 5 January 2015 (UTC)


 * Ah, I got it! I wasn't pushing the value back onto the stack again. So it is:
 * NOT(((D xor P) or (D xor S)) xor S)
 * I had to reread that algorithm on the wiki a few times. - Letsbefiends (talk) 02:25, 5 January 2015 (UTC)


 * Cool, I'll mark this Q resolved then. StuRat (talk) 03:16, 5 January 2015 (UTC)
 * Well, RPN is to make it easy for the machine, not for people. :-) Bubba73 You talkin' to me? 04:24, 5 January 2015 (UTC)


 * Presumably that first "D" in your answer was a typo, and you meant to state:
 * not(((S xor P) or (D xor S)) xor S)
 * -- ToE 06:04, 5 January 2015 (UTC)

Here is how a human might resolve this, looking at the expression (and rewriting if necessary) by iterative replacing operators with their respective arguments: CiaPan (talk) 05:52, 5 January 2015 (UTC)
 * 1) SSP xor DS xor or xor not
 * 2) S (S xor P) (D xor S) or xor not
 * 3) S ((S xor P) or (D xor S)) xor not
 * 4) (S xor ((S xor P) or (D xor S)) not
 * 5) not (S xor ((S xor P) or (D xor S))


 * There's even a programming language based on this Forth (programming language) which used to be reasonably popular. It has often been described as a Write-only language ;-) Dmcq (talk) 16:32, 5 January 2015 (UTC)

Fermat point of spherical triangles
Where is it? (Defining it analogically as the point where the sides subtend equal angles.) Double sharp (talk) 08:32, 5 January 2015 (UTC)
 * The article on Fermat point would say something about it if there was anything so it's unlikely there is anything on Wikipedia. There;s a couple of papers on the web if you Google 'Fermat point sphere'. An opportunity to contribute to Wikipedia by adding to that article? Dmcq (talk) 16:43, 5 January 2015 (UTC)
 * Hmm, this looks promising. Double sharp (talk) 21:45, 17 March 2016 (UTC)