\( f(x) = \begin{cases} x_b \text{ if } x < x_b-a \\ x_b + ((35 a^3 - 47 a^2 (x - x_b) + 25 a (x - x_b)^2 - 5 (x - x_b)^3) (a + x - x_b)^5)/(256 a^7) \text{ if } |x-x_b| < a \\ x \text{ if } x > x_b + a \end{cases}\) The integral of PolynomialHeaviside approximates xH(x)
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\( f(x) = \begin{cases} x_b \text{ if } x < x_b-a \\ x_b + ((35 a^3 - 47 a^2 (x - x_b) + 25 a (x - x_b)^2 - 5 (x - x_b)^3) (a + x - x_b)^5)/(256 a^7) \text{ if } |x-x_b| < a \\ x \text{ if } x > x_b + a \end{cases}\) The integral of PolynomialHeaviside approximates xH(x)
This function is 4 times continuously differentiable,
has a transition width \c a on both sides of \c xb, where it transitions from the
constant \c xb to the linear function \c x.