  Strong-coupling expansion is performed for the lattice phi^4 model in 1+1
dimensions. Because the strong-coupling limit itself is not solvable, we
employed numerical calculations so as to set up unperturbed eigensystems.
Restricting the number of Hilbert-space bases, we performed linked-cluster
expansion up to eleventh order. We carried out alternative simulation by means
of the density-matrix renormalization group. Thereby, we confirmed that our
series-expansion data with a convergence-acceleration trick are in good
agreement with the simulation result. Through the analytic continuation to the
domain of negative biquadratic interaction, we obtain the false-vacuum decay
rate. Contrary to common belief that tunnelling phenomenon lies out of
perturbative treatments, our series expansion reproduces the instanton-theory
behaviour for high potential barrier. For shallow barrier, on the contrary, our
result tells that the relaxation is no more described by instanton, but the
decay rate acquires notable enhancement.
