Singular Manual: backward

| #### D.15.4.7 backward Procedure from library `finitediff.lib` (see finitediff_lib). Usage: backward(U1,U2,var); U1, U2 are the names of occuring derivatives, var is a variable in the basering; Return: type vector; gives a predefined approximation of the backward approximation as often used in literature; Note: see also forward,laxfrT,setinitials,scheme; Example:
    LIB "finitediff.lib";
    list D="Ut","Ux","Uy","U";
    list V="t","x","y";
    list P="a","b";
    setinitials(V,D,P);
    backward(Ux,U,x);
    ==> [0,(dx)*x,0,-x+1]
    backward(Uy,U,y);
    ==> [0,0,(dy)*y,-y+1]
    backward(Ut,U,t);
    ==> [(dt)*t,0,0,-t+1]

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