  We consider the virtual corrections to Higgs boson pair production at
next-to-next-to-leading order, in the large top quark mass limit. We compute
five expansion terms for the box-type form factors and eight expansion terms
for the triangle form factor, which serve as useful input for the construction
of approximations. We present analytic results for the form factors in the
soft-virtual approximation. From a technical point of view the calculation is
quite challenging since huge intermediate expressions are produced. We describe
our methods and optimizations to overcome these difficulties, which might be
useful for other calculations.
