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- public interface interface
**BiLinearOperatorHessian** - extends JamObject, BiLinearOperatorAdjoint, hasHessian

The derivative of a bilinear operator B(x,y) at a point [x0 y0]' (where we consider B(x,y) as a non-linear (unary) operator of [x y]') is the row operator:

[B(.,y0) B(x0,.)]

That is:

Derivative at [x0 y0]' acting on [x y]' = B(x,y0) + B(x0,y)

The Hessian of a bilinear operator considered as a unary operator is very simple:

Hessian([x y]', [v w]') = B(x,w) + B(v,y)

The Hessian is independent of its place of evaluation ([x0 y0]'). In fact, not surprisingly, the full Taylor expansion of B ends with the Hessian, i.e. there are no terms higher than second order.

The bilinear property of B gives us the Taylor expansion immediately:

B(x+x0, y+y0) = B(x0,y0) + {B(x,x0) + B(x0,y)} + (1/2){2B(x,y)}

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