DATE     97/08/12
    Opérateur ACT3
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     CHPO  =  ACT3 CHPO2 CHPO1 CHPO0 CHFO3 CHFO2 CHFO1 CHFO0;

    Description :
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    The ACT3 operator is used for accelerating the convergence of
 non linear calculations in mechanics.

    Method :
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   From 3 increments of correcting forces and the 4 corresponding
imbalance forces, ACT3 evaluates the correction of correcting force
allowing to minimize imbalance by solving the tangential problem
on it's restriction to the subspace of dimension 3 defined by the 
fields on input.