# [Om] usage of OM symbols for physical units

Christian Groß gross at math.uni-augsburg.de
Mon May 29 11:26:46 CEST 2006

```Hello everybody,

I have a question on the OpenMath symbols for physical units, in
combination with their prefixes, i.e., on the cd's
units_metric1 and units_siprefix1.
In units_metric1 you have symbols metre and metre_sqrd and in
units_siprefix1 you find prefixes like centi and milli.

I guess this works well in order to express cm = centi-metre or mm =
milli-metre. But what about cm^2 as a small unit for area?
One could construct "(centi-metre)^2" of course, using the usual power
symbol, etc. But if it is intended that for defining physical symbols
one needs the power symbol anyway, then which sense has the symbol
metre_sqrd? It should be (metre)^2 in this case, if the whole system has
a systematic. On the other hand, I could also use centi-metre_sqrd, but
in my opinion this would mean 10^(-2)*m^2, whereas cm^2 is 10^(-4)*m^2.

Similar questions arise for the units of volumes. Why is there a "litre"
symbol but no "metre_cubic"?

Finally, without giving any reference to the special case "kilo", the
description of "prefix" in units_ops1 doesn't make much sense:

prefix

Description:
"This symbol represents the fact that the subsequent unit has been
effectively multiplied by 1,000 (\$10^{3}\$)"

Commented Mathematical property (CMP):     This is unit multiplication
by 1,000

Thanks for any help and comments,

regards,
Christian Gross

```