Solving the TVM equation for the interest rate

[Ángel Martin]

I looked into the very same subject when I wrote the HP-41 MCODE version of TVM$.

I don't recall exactly how or why I came to this, but the expression I used for the initial guess was:

i0 = [ abs( PV + n*PMT + FV) ]^1/n

Surely this isn't the best, or may be not even sensible - but it works just fine for all tests and still holding up nicely.