Renormalized sextic coupling constant for the two-dimensional Ising   model from field theory
  The field-theoretical renormalization group approach is used to estimate the universal critical value g_6^* of renormalized sextic coupling constant for the two-dimensional Ising model. Four-loop perturbative expansion for g_6 is calculated and resummed by means of the Pade-Borel-Leroy technique. Under the optimal value of the shift parameter b providing the fastest convergence of the iteration procedure the estimates g_6^* = 1.10, g_6^*/{g_4^*}^2 = 2.94 are obtained which agree quite well with those deduced recently by S.-Y. Zinn, S.-N. Lai, and M. E. Fisher (Phys. Rev. E 54 (1996) 1176) from the high-temperature expansions. 
A. I. Sokolov and E. V. Orlov (Saint Petersburg Electrotechnical   University, St. Petersburg, Russia)