!copyright (C) 2001  MSC-RPN COMM  %%%RPNPHY%%%

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**  fonctions de couche de surface pour le cas stable                 **
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*
      d  (unsl) = 4*AS*BETA*unsl
      c  (hi)   = d(unsl)*hi - hi**2
      b  (hi)   = d(unsl) - 2*hi
      a  (z,hi) = sqrt(1 + b(hi)*z - c(hi)*z**2)
      psi(z,hi) = 0.5 * (a(z,hi)-z*hi-alog(1+b(hi)*z*0.5+a(z,hi))-
     +            b(hi)/(2*sqrt(c(hi)))*asin((b(hi)-2*c(hi)*z)/d(unsl)))
*
*   Limites de validite: unsl >= 0 (cas stable ou neutre)
*                        c > 0 (hi < d)
*                        z*hi < 1
*   Ces 2 dernieres conditions imposees a l'aide du facteur 'factn'
*
*   Reference :  Y. Delage, BLM, 82 (p23-48) (Eq.33-37)
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*