2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/26987We obtain the variational upper bound for the ground- state energy of two-dimensional antiferromagnetic Heisenberg model on a square lattice at arbitrary value of the anisotropy parameter using the two-dimensional generalization of Jordan-Wigner transformation. Our result can be considered as an upper bound for the perturbation theory series about the Ising limit.7 pages, LaTex, JETP Letters (Pisma v ZhETF) 57 (1993) 796-800Condensed Mattertext