Private Sub Command1_Click()
    'PROGRAM D14R1
    'Driver for routine RK4
    N = 4
    Dim Y(4), DYDX(4), YOUT(4)
    X = 1#
    Y(1) = BESSJ0(X)
    Y(2) = BESSJ1(X)
    Y(3) = BESSJ(2, X)
    Y(4) = BESSJ(3, X)
    DYDX(1) = -Y(2)
    DYDX(2) = Y(1) - Y(2)
    DYDX(3) = Y(2) - 2# * Y(3)
    DYDX(4) = Y(3) - 3# * Y(4)
    Print Tab(5); "Bessel Function:   J0   J1   J2   J3"
    For I = 1 To 5
        H = 0.2 * I
        Call RK4(Y(), DYDX(), N, X, H, YOUT())
        Print
        Print Tab(5); "For a step size of: "; Format$(H, "#.#0")
        Print Tab(5); "RK4:      ";
        For J = 1 To 4
            Print Format$(YOUT(J), ".#####0"),
        Next J
        Print Tab(5); "Actual:",
        Print Tab(15); Format$(BESSJ0(X + H), ".#####0");
        Print Tab(29); Format$(BESSJ1(X + H), ".#####0");
        Print Tab(43); Format$(BESSJ(2, X + H), ".#####0");
        Print Tab(57); Format$(BESSJ(3, X + H), ".#####0")
    Next I
End Sub
Sub DERIVS(X, Y(), DYDX())
    DYDX(1) = -Y(2)
    DYDX(2) = Y(1) - (1# / X) * Y(2)
    DYDX(3) = Y(2) - (2# / X) * Y(3)
    DYDX(4) = Y(3) - (3# / X) * Y(4)
End Sub