Jonathan Knibb wrote:

>Rather than give the whole system of rules, maybe it would be more fun
>just to list the first few numbers and let people work it out.

Quite the system! I've managed to discover the rules: 1-13, and 15 and 17,
have their own names.
Postposed _ala_ means "times two", _efen_ "times three", _ilcur_ "times
four", and _ilku_ "times five". There is also one instance of _eldena_
which means "times six". The "phonological alternations" you mentioned are
evident in these words: ra - ala, pen - efen, sur - ilcur, chu - ilku.
This takes care of most of the numbers.
The leftovers follow some odd rules. _ru_ seems to mean a function 2x+1:
_ru ildivi_ "two times divi (9), plus one" is 19; _ru alvami_ "two times
vami (15), plus one" is 31, etc. There are also compounds with _ru_:
38 - ru ilděvi alá = ("ru" 9) times two
43 - ru eso efén = "ru" 21 [21 being _co efen_]
46 - ru allŕli alá = ("ru" 11) times two
47 - ru ilu alláli = "ru" ("ru" 11); that is, "ru" 23.
Another function is found in _ca_, which is 4x+1: _ca ildivi_ "four times
divi (9), plus one".
So all that is left is 25, _chu ede_. Since _chu_ is 5, I'd guess this is
simply "five squared".

>(And who can tell me what 1918 comes to in this system?  :)) )

I can't figure that one out. Or perhaps I can with more time, but I really
want to post this first!