Our current knowledge of the quark–gluon plasma in thermodynamical equilibrium is
reviewed. The phase diagram of strongly interacting matter is discussed, with emphasis on …

An optimized perturbation theory (OPT) at finite temperature T, which resums higher order
terms in the naive perturbation, is developed in O (N) φ 4 theory. It is proved that (i) the …

We use the recently developed variational perturbation theory to continue the
renormalization constants of three-dimensional φ 4 theories to the regime of strong bare …

The convergence of the linear δ expansion is studied in the context of the integral I:= F−∞∞
e− gx 4 dx, which corresponds to massless cphi 4 theory in 0 dimensions. The method …

We prove that the linear delta expansion for energy eigenvalues of the quantum mechanical
anharmonic oscillator converges to the exact answer if the order dependent trial frequency Ω …

The optimized linear δ expansion is applied to multifield O (N 1)× O (N 2) scalar theories at
high temperatures. Using the imaginary time formalism the thermal masses are evaluated …

We develop an efficient algorithm for evaluating divergent perturbation expansions of field
theories in the bare coupling constant g B for which we possess a finite number L of …

Variational perturbation expansions have recently been used by the authors to calculate
directly the strong-coupling expansion coefficients of the anharmonic oscillator. The …

The perturbation theory with a variational basis is constructed and analyzed. The
generalized Gaussian effective potential is introduced and evaluated up to second order for …

The optimized linear δ expansion is applied to the λ φ 4 theory at high temperature. Using
the imaginary time formalism the thermal mass is evaluated perturbatively up to order δ 2. A …