Strong-coupling behavior of ${\phi }^4$ theories and critical exponents

We use the recently developed variational perturbation theory to continue the renormalization constants of three-dimensional ${\phi }^4$ theories to the regime of strong bare coupling $g_0.$ In this limit, they are found to behave as powers in $g_0,$ thereby determining all critical exponents. Convergent strong-coupling expansions are found for all renormalization group functions which converge well even in the weak-coupling limit.