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## Expressions
In order to write a differential equation in a mathematical natural way we have developed an expression framework, that allowes exacly this. Instead of adding abstract operator-classes to the problem definition we implement the coefficient functions of the operators using mathematical operators. An example is the following bilinearform
<!-- begin MathToWeb -->
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<mrow>
<mi>a</mi>
<mo maxsize="1">(</mo>
<mi>c</mi>
<mo>,</mo>
<mi>ϑ</mi>
<mo maxsize="1">)</mo>
<mo>:</mo>
<mo>=</mo>
<mo>〈</mo>
<mfrac>
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<mn>1</mn>
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<mrow>
<mi>ϵ</mi>
</mrow>
</mfrac>
<mo maxsize="1">(</mo>
<msup>
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<mi>ϕ</mi>
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<mrow>
<mn>2</mn>
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<mo>-</mo>
<mn>1</mn>
<mo maxsize="1">)</mo>
<mi>c</mi>
<mo>,</mo>
<mspace width="0.278em"></mspace>
<mi>ϑ</mi>
<mo>〉</mo>
<mo>+</mo>
<mo>〈</mo>
<mo>max</mo>
<mo maxsize="1">(</mo>
<msup>
<mrow>
<mn>10</mn>
</mrow>
<mrow>
<mo>-</mo>
<mn>5</mn>
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</msup>
<mo>,</mo>
<mspace width="0.278em"></mspace>
<mo maxsize="1">(</mo>
<mi>ϕ</mi>
<mo>+</mo>
<mn>1</mn>
<mo maxsize="1">)</mo>
<mo maxsize="1">)</mo>
<mo>∇</mo>
<mi>c</mi>
<mo>,</mo>
<mspace width="0.278em"></mspace>
<mo>∇</mo>
<mi>ϑ</mi>
<mo>〉</mo>
</mrow>
</math>
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<img src="/uploads/8d0d247b4047613ff6bd9e75afa4ab6d/formula.png" height="50" title="a(c,\vartheta) := \big\langle\frac{1}{\epsilon}(\phi^2 - 1) c,\; \vartheta\big\rangle + \big\langle\max(10^{-5},\;(\phi+1))\nabla c,\; \nabla\vartheta\big\rangle" />
Assume that &phi; represents a DOFVector, i.e. a discrete representation of a function in a function-space, that is known in advance, or given by an iterative solution procedure from the last iteration. Here we use a solution component of the problem `prob`:
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