  In this paper it is shown that higher order quasiconvex functions suitable in
the variational treatment of problems involving second derivatives may be
extended to the space of all matrices as classical quasiconvex functions.
Precisely, it is proved that a smooth strictly 2-quasiconvex function with
p-growth at infinity, p>1, is the restriction to symmetric matrices of a
1-quasiconvex function with the same growth. As a consequence, lower
semicontinuity results for second-order variational problems are deduced as
corollaries of well-known first order theorems.
