Informal description
Let \( E \), \( F \), and \( G \) be normed spaces over a field \( \mathbb{K} \). For any continuous bilinear map \( f : E \to_{L[\mathbb{K}]} F \to_{L[\mathbb{K}]} G \) and any point \( x \in E \times F \), the function \( (x_1, x_2) \mapsto f(x_1)(x_2) \) has a formal power series expansion \( f.\text{fpowerSeriesBilinear} \, x \) centered at \( x \) with an infinite radius of convergence. Specifically, for all \( y \) in the ball of infinite radius centered at \( x \), the function can be expressed as:
\[ f(x_1 + y_1, x_2 + y_2) = \sum_{n=0}^\infty (f.\text{fpowerSeriesBilinear} \, x)_n (y_1, y_2)^n. \]
Here, the series terminates after the second term, with the 0-th term being \( f(x_1, x_2) \), the 1-st term being the derivative of \( f \) at \( x \), and the 2-nd term being the bilinear map \( f \) itself, while all higher terms vanish.