using local update rules you don't have to do back prop and that's what this algorithm is proposing namely if you think of a stack of autoencoders this this this transforming one hidden representation into the next right this is the feed-forward function what you can do is you first of all you can assume that for each of these functions here you have a perfect inverse right you can you can perfectly compute the inverse function that's this this G here of course this doesn't exist but assume you have it what you then could do is you could if