The discounted sum of a project's present and future

cash flows. In theory all projects with positive a NPV should be

viable, though it is common experience that this is not the case. The

interest rate (

`r`

) where the NPV is zero is

internal rate of return (IRR).

NPV is generalized:

Σ`c`

_{t}/(1+`r`_{t}

)^{t} for `t`

= 1..n

`t`

- is the zero-base number of the period
`c`

- is the signed cash flow
`r`

- is the interest rate for period t