Formula for entropy conditional on >1 variables?

[Inquirer]

Hello,

I want to compute the conditional entropy H(A | B,C), given probability distributions for each of the variables.

It would be nice to have a right-hand side not involving conditionals. H(A|B) = H(A,B) - H(B) but how does it work out if there are more than one conditional variable?

I tried, as a dumb guess, H(A,B,C) - H(B,C), but from using it further down the line, I doubt that's correct.

Thanks!

 

[Inquirer]

I worked through it again and discovered an error or two in my calculations. In the end it looks like

H(A|B,C) = H(A,B,C) - H(B,C)

is correct after all.

This is quite clear when illustrated with a Venn diagram similar to the one https://secure.wikimedia.org/wikipedia/en/wiki/Conditional_entropy" .