ODFF: IPMT, PPMT

["David King" <openoffice@drking.plus.com>]

IPMT: "TODO: which is correct?"

I think
=IPMT(5%/12;10;360;100000;0;1)
is correct in Calc and Excel, if you know what it is trying to 
calculate.

The problem arises because Excel's help describes this as:
"Returns the interest payment for a given period ..."
http://office.microsoft.com/en-us/excel/HP100623641033.aspx
which is *not* what it returns.
It returns 'the portion of the periodic payment which is 
interest' - so when the payment is at the beginning of the period 
the interest is calculated in the period before that. The first 
period payment is at the very start and has zero interest.

I have derived the formulae and they agree with Excel and Calc. 
It's more than possible that Gnumeric is implementing what 
Excel's help says it should. Gnumeric's code is simple enough, 
but I haven't waded through the algebra to confirm; the help 
suggests it though.

PPMT: "TODO: To add the formula definition. I can not find it 
yet."

The formula is simply PMT-IPMT, and that's how Calc at least 
works it out.

Hm, shouldn't I be doing something else...?
D