  Exact numerical results of the interacting boson model Hamiltonian along the
integrable line from U(5) to O(6) are obtained by diagonalization within boson
seniority subspaces. The matrix Hamiltonian reduces to a block tridiagonal form
which can be diagonalized for large number of bosons. We present results for
the low energy spectrum and the transition probabilities for systems up to
10000 bosons, which confirm that at the critical point the system is equally
well described by the Bohr Hamiltonian with a $\beta^4$ potential.
