Hi. Usually in set theory, Union[{1,b,d},{1,b,a}] = {1,b,d,a}]. In other words, only unique values are passed. In other words, the ordering information is lost. But for my physics problem, I need Union[{1,b,d},{1,b,a}] = {1,b,d,1,b,a}. Can someone please tell me how I would denote this type of union. In Mathematica, this would be Joining two lists. But is there a more concise mathematical notation.

Thanks.