Is there a Math API for Pow(decimal, decimal)

One of the multipliyers is a rate : 1/rate^(days/365).

The reason there is no decimal power function is because it would be pointless to use decimal for that calculation. Use double.

Remember, the point of decimal is to ensure that you get exact arithmetic on values that can be exactly represented as short decimal numbers. For reasonable values of rate and days, the values of any of the other subexpressions are clearly not going to be exactly represented as short decimal values. You're going to be dealing with inexact values, so use a type designed for fast calculations of slightly inexact values, like double.

The results when computed in doubles are going to be off by a few billionths of a penny one way or the other. Who cares? You'll round out the error later. Do the rate calculation in doubles. Once you have a result that needs to be turned back into a currency again, multiply the result by ten thousand, round it off to the nearest integer, convert that to a decimal, and then divide it out by ten thousand again, and you'll have a result accurate to four decimal places, which ought to be plenty for a financial calculation.

Here is what I used.

output = (decimal)Math.Pow((double)var1, (double)var2);

Now I'm just learning but this did work but I don't know if I can explain it correctly.

what I believe this does is take the input of var1 and var2 and cast them to doubles to use as the argument for the math.pow method. After that have (decimal) in front of math.pow take the value back to a decimal and place the value in the output variable.

I hope someone can correct me if my explination is wrong but all I know is that it worked for me.

Well, here is the Wikipedia page that lists current C# numerics libraries. But TBH I don't think there is a lot of support for decimals

http://en.wikipedia.org/wiki/List_of_numerical_libraries

It's kind of inappropriate to use decimals for this kind of calculation in general. It's high precision yes - but it's also low range. As the MSDN docs state it's for financial/monetary calculations - where there isn't much call for POW unfortunately!

Of course you might have a specific problem domain that needs super high precision and all numbers are within 10(28) - 10(-28). But in that case you will probably just need to write your own series calculator such as the one linked to in the comments to the question.