Mapping a triplet of numbes

[Guest]

Hi! I am a physics student and creating a code I found the following problem.
I have to be able to map a triplet of numbers into an integer that goes from 1 to a certain value.
In this case I have the triplet {n, l, m} and I must make the mapping of these numbers go from 1 to [tex]{n+l+m+1 \choose 2}[/tex].
It is very important not skip numbers and that the numbers are in the range mentioned above.
I hope you can help me (excuse my English, I write in a translator)
Thank you!