public IOperator add(IOperator A, IOperator B)
{
TridiagonalOperator D1 = A as TridiagonalOperator;
TridiagonalOperator D2 = B as TridiagonalOperator;
Vector low = D1.lowerDiagonal_+D2.lowerDiagonal_,
mid = D1.diagonal_+D2.diagonal_,
high = D1.upperDiagonal_+D2.upperDiagonal_;
TridiagonalOperator result = new TridiagonalOperator(low,mid,high);
return result;
}