R.I. N¡ 279 J. NEVEU THE EXPONENTIAL OF THE SQUARE 4 MODULE OF A DIFFUSION The whole paper is available as a compressed Postscript file by internet procedure FTP anonymous on host barbes.polytechnique.fr ( 129.104.4.100) in the directory pub/RI/1993 under the name neveu.279.avr.93.ps.gz