Does anyone have the definitive formulae behind the excel PMT, IMPT and PPMT functions?
I've found very few but varying examples across the web.
I've looked these up, as needed, and implemeted them myself in a simple micro-service.
IRR, for example, though not specifically requested by you (shown in the top example below) was straightforward to implement. If you set up a micro-service, you can call that code with a single INSERT FROM URL line of code.
For more information about micro-services, see my tutorials at:
Create Micro-Services Using Java and the Spark Java Framework
The Simplest Micro-Service! (Python and Flask)
BTW, I'd be more than happy to help you get these functions working in your micro-service (that you could also then use from any other HTTP-enabled application, like the Browser, not just FMP).
Retrieving data ...