[marcosdumay]

Well, the first theory is by Keynes, it's on his book.

It's been revised and remodeled several hundreds of times, if you want to search Google Schoolar or something like that, look for "demand side" theories, or crisis of "aggregated demand". Also, if you search for NGDP, the biggest share of the papers will be about this.

There are many other terms that will lead to less results. It's a really widespread concept.

[tarikjn]

Mainstream yes, as in the majority believe so. But not without controversy. The Austrian School of economics is an opposing view which has brought just as many results of being the correct philosophy, and one may argue, is a simpler and more elegant approach of economics. And as one may say, clear ideas are easily expressed.