FU XI-QUAN, GUO HONG, HU WEI, LIU CHENG-YI, WANG XIAO-PING. INVALIDATION OF THE SLOWLY-VARYING ENVELOPE APPROXIMATION AND PRODUCING AND ELIMINATING OF THE SPATIAL SINGULARITY FOR ULTRASHORT PULSED-BEAM PROPAGATION IN FREE SPACEJ. Acta Physica Sinica, 2001, 50(9): 1693-1698. DOI: 10.7498/aps.50.1693