I need some help with this one:

A permutation is indecomposable if it cannot be cut

into two parts so that everything before the cut is smaller than everything

after the cut. For example 3142 is indecomposable, but 2143 is not as you

can cut it after the first two elements. Let f(n) be the number of indecomposable

permutations of length n and set f(0) = 0. Find the ordinary generating

function.