// check format of FiniteDifference.ToString()
public void CheckToString()
{
// check zero in the middle and signs and (x+h)
FiniteDifference difference = FiniteDifference.GetFiniteDifferenceByOrderAndMinH(3, -2) -
3 * FiniteDifference.GetFiniteDifferenceByOrderAndMinH(0, -1);
string goodString = "u(x+h)-3*u(x)-u(x-2*h)";
Assert.Equal(goodString, difference.ToString());
// check leading -, (x-h), (x+2*h)
difference = -2 * FiniteDifference.GetFiniteDifferenceByOrderAndMinH(3, -1);
goodString = "-2*u(x+2*h)+6*u(x+h)-6*u(x)+2*u(x-h)";
Assert.Equal(goodString, difference.ToString());
}