Talk:Relation algebra

message: Apparently, this article uses unconventional notation, and it certainly seems crazy to me. Why not use the conventional notation? -Chinju 05:31, 28 March 2007 (UTC)
replies: text: I second the motion. If we have to use parenthesis to represent boolean negation as in -x=(x), how do we use parenthesis normally in an equation? I suppose that we could express the associative law of addition (union) as ((x+y))+z=x+((y+z)) on the theory that the double parenthesis represents a double negation. As long as we double up on all of our parenthesis, we ought to be safe. ;o) --Ramsey2006 02:38, 29 March 2007 (UTC)
replies: text: Ambiguity arising from infix notations are a bane of algebra and computer science. Polish prefix notation has not caught on, but there is another way of banishing ambiguity without using brackets. If a binary operation associates, it is not necessary to establish the order in which multiple instances of the same operation are carried out; simply concatenate the operands and enclose them in a prespecified pair of brackets. The two primitive binary operations of RA, Boolean join and composition, both associate by virtue of axioms B2 and B4.132.181.160.42 (talk) 23:24, 24 May 2009 (UTC)
thread_title: Older comments

message: The article seemed to assert (by citation) that the notions of injectivity, surjectivity were invented in 2002. These notions are indeed mentioned briefly in the referenced article, but they were well-known for years before then. I have removed these citations. I've left a (partial) reference to that article at the end of the page. I'm not sure that it is an important general reference, but I leave it to others to decide whether to delete it. 
If I have missed an important point, I'm sorry. I'm happy to discuss. Sam Staton 09:35, 27 September 2007 (UTC)
thread_title:  Removed strange links 

message: RA is a class of relational algebras, not the class of equations valid on that class of algebras, i.e. it is not some equational theory. The section should be talking about how the expressive power of some fragment of a first-order language (not logic!) corresponds to an equational language for RA. In particular, such first-order formulas are satisfied in a model iff the equation is "verified" in the corresponding relational algebra under the appropriate valuation. This section of the article currently makes little sense. Nortexoid (talk) 19:39, 19 August 2008 (UTC)
replies: text: Agreed.  That section was left over from before my rewrite.  I didn't have the heart to delete it, but it certainly needs a thorough rewriting if it is to be kept.  --Vaughan Pratt (talk) 00:52, 19 January 2009 (UTC)
thread_title:  Expressive power section is poor 

message: It looks like this page uses \neg A in some places and A^- in others. AshtonBenson (talk) 01:17, 10 May 2010 (UTC)
replies: text: Good catch (mea culpa).  Hopefully I fixed them all, let me know if not.  --Vaughan Pratt (talk) 08:10, 26 May 2010 (UTC)
thread_title:  Inconsistent Notation 

message: Our article on asymmetric relation defines it as a relation where a R b holding implies b R a does not hold. It seems evident that this is not the same thing as R ≠ R^\breve{\ }, which is merely the negation of the expression R = R^\breve{\ } for a symmetric relation. Many relations are neither symmetric nor asymmetric, including e.g. ≤. Can someone double check this? Dcoetzee 12:20, 21 August 2013 (UTC)
thread_title:  Incorrect expression for asymmetric relations? 

message: Someone should add the plainest, simplest possible example in predicate logic of a four-variable principle, definition or theorem (maybe plainer than general associativity of relative product and relative sum, if there is one) that can't be done in the algebra.  (Tarski & Givant) Ideally, provide it in an English sentence that Joe Blow (in Britain, Fred Bloggs) can understand.  The current remark about it leaves the reader hanging. 108.73.1.253 (talk) 20:42, 11 December 2017 (UTC)
thread_title:  Add a four-variable counterexample, as simple as possible 

message: Many links are outdated. Please refer to de:Relationsalgebra for fixing as all links have been reviewed there. --Ernsts (talk) 00:56, 27 January 2018 (UTC)
thread_title:  Broken Weblinks