Program SIMPSON_3VAR
        IMPLICIT DOUBLE PRECISION (A-H,O-Z)
        call cpu_time(start)
        a=0d0
        b=1d0
        c=(a+b)/2d0
        r=(b-a)/6d0*(g3(a)+4d0*g3(c)+g3(b))

        open(20,File="ejerc10simp3var.xls")

        write(20,*)r
        call cpu_time(finish)
        write(20,*)"Temps = ",finish-start ,"s"
        close(20)
        END


        FUNCTION  g1(x1,x2,x3)
        Implicit Double Precision (A-H,O-Z)
        g1=exp(-(x1*x1)-(x2*x2)-(x3*x3))*cos(x1*x2+x2*x3+x1*x3)
        END
        
        Function g2(x2,x3)
        IMPLICIT DOUBLE PRECISION (A-H,O-Z)
        a=0d0
        b=1d0
        c=(a+b)/2d0
        g2=(b-a)/6d0*(g1(a,x2,x3)+4d0*g1(c,x2,x3)+g1(b,x2,x3))
        END
        
        Function g3(x3)
        IMPLICIT DOUBLE PRECISION (A-H,O-Z)
        a=0d0
        b=1d0
        c=(a+b)/2d0
        g3=(b-a)/6d0*(g2(a,x3)+4d0*g2(c,x3)+g2(b,x3))
        END