[Om] usage of OM symbols for physical units

[W Naylor]

On Wed, 26 Jul 2006, Paul Libbrecht wrote:

> W Naylor wrote:
> > I had corespondance with Professor Davenport in 2002, we finally resolved
> > that the arith1 symbols would be used, there is a discussion document on
> > this at:
> > http://www.openmath.org/cocoon/openmath/documents/Units.pdf
> > 
> 
> I have a further issue with the usage of the arith1/times symbol which I find
> quite big: if one wishes to use variables for units, as is done in the example
> for units_prefix, the we have now way to see that x*y is actually x units-y
> instead of x*y except from some context information about y.

The example given in units_prefix, is intended to be a generalisation 
were the variable 'unit', represents a unit, i.e. it may be a second 
(from units_time1), a metre (from units_metric1), etc.. If one wants to 
use a variable to represent the unit, then as I see it, there are 
three possibilities:

1) it is a general observation applicable over all units
2) it is an incorrect statement (at least in some interpretations)
3) the unit should be attributed with a (dimension) type using an OMATTR 
element, (I realise there is some argument as to whether OMATTR elements 
must be respected but I don't think this applies here).

> 
> Why not define units_ops1/unittimes as a symbol which specializes arith1/times
> (at least verbally)??

This was considered (back in 2002) but not deemed necessary. 
One problem I see here is more to do with typing of arguments (to the 
operator). For example, if one says 10*metres, this is effectively giving 
the operator '*' two arguments, the first a dimensionless quantity and the 
second a quantity with the dimension of length. We just decided to ignore 
the sts constraints (or at least overload the operators with a sense not 
explicated in the sts files) and use them in the way stated in the 
document you site. 

I hope this helps,

cheers,

Bill

> 
> paul
> 

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-                  Dr. W.A. Naylor
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-                  http://orcca.on.ca/~bill
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